I am ballparking here but i think a good rule of thumb for buying speed.....with all things being equal..
$500 gets you 1 sec per KM. over the next cheapest bike.

does that sound resonable?

Once you pay about $600 for a road bike, there is no difference the bike is going to make. A $10k bike will not make you appreciably faster in any group ride on the road than this bike:
https://www.bikesdirect.com/products/.../orion_al8.htm

Now, in a time trial, aero will make a difference (although the aero helmet makes a much bigger diference than the bike and wheels). If you did the pursuit, back to back on different days, once with a plain road bike and another with a purpose built pursuit bike, you would easily see several seconds difference.

Not sure how you can say that when you don't even know the weight of that bike... If it's 35lbs ready to ride, I can guarantee my 25lb road bike is a lot faster, especially up hills!

The single biggest factor is, and always will be...the engine.

For us clydes, as long as you have a bike that is mechanically sound and not always in the shop, and weighs less than 30 lbs or so, and fits you such that you don't hurt your knees or the nerves in your hands, the biggest thing that speed will cost you is time.

I have to spend a lot of time in the saddle to get faster. For me that means that I pay in other ways besides money. That means that I get up at 4:30 AM 3-4 times per week. That means that my back yard looks like a weed-infested nightmare to my neighbors. That my dogs don't get as much play time at the park as they normally would. That most of my other hobbies pretty much vanish.

It is worth it though. I love being able to actually enjoy long rides in the country without having to take half the day to do it.

+10000 on the engine. I, daily, pass people on roadies. The guy yesterday on the $4,000 carbon Trek MTB that I blew past was NOT happy to get passed by a fat dude on a big steel touring trucker.

Totally. Buying Lance's bike doesn't mean that you can win the Tour de France.

The price of speed is a lot of work and training.

Buying speed, with all other things being equal, means stripping weight from the bike.

Stripping weight from the bike is exponentially more expensive as you get lighter.

Going from 30 to 25 pounds can be done for under $500.
25 down to 20 pounds might mean another $1000 increase.
Dropping another 3 pounds to 17 could cost yet another $1000.
You wanna shave those last 100 grams off the bike? There's another $1000.

no i think you missing my point.

On my Fuji touring i avg 30 kph or a 2 min KM
if i where to jump on e a madone for the next KM I think i should expect the next kilomete to be 1 min. 54 secs. with the thumbrule of 1 sec per $500.

Pain and suffering. Lot's of it.

Incorrect. Yes, all other factors being equal, going from a 30lb touring bike to a 20lb road bike may make the ride go a little faster...weight being only one factor. But the thing is, I could easily spend 10k on a primo touring bike, but still be faster on a 1k road bike...so price is not the issue unless you are comparing apples to apples.

OK, so let's say we are comparing an entry level road bike with higher end road bikes (apples to apples). Sure, you may see a difference going from a $600 bike to a $1.2k bike to a 2.4k bike, but as you go up in price the difference in performance becomes much less evident...so the increases are not linear.

At 270 pounds, I think my bike is the least of my problems. lol

QFT, skipping pizza the night before makes a much bigger difference to me.

Easy, the $600 alu frame with carbon fork is going to weigh somewhere in the 20 lbs range, a $10k bike is going to weigh about 16 pounds. 4-5 pounds difference will make no appreciable difference in a group and no apprecdiable difference in a time trial. Aero makes the biggest difference, not weight.

If you can show me a $600 road bike that weighs 20 real-world pounds in ready-to-ride form, I'll buy one on the spot! Heck, if you can show me a $600 road bike that weighs less than 25 real-world pounds in ready-to-ride form, I'll buy one...

A) I doubt that bike sits in the 20 pound range.
B) You can go sub 14lbs for less than 6K. Verrrrrry few people are going to spend 10 grand on a 16lb bike.
C) I ride in rather hilly terrain, less weight absolutely makes a difference.

The OP needs to quantify "appreciable" to have this discussion.

Unless it's an uphill TT!...My wife rides her 17 'ish cf roadie and does much better on a 3,000 ft climb than she does on her old steel Bianchi


But the OP is correct! I (before I knew better) was interested in some fancy liteweight aero wheels. Good thing I had a decent salesman. He said the $1200 would only save me about .3 of a second in a 20 mile ride. Not for me!

Well, not really, there are strict limits into how much improvement you gain by spending money on equipment, there is a sweat spot on equipment investment, I think right now it's around $1000, beyond that you double the cost for half the improvement. Real speed improvements are like race cars, they are done to the engine.

What you need for real speed are several things:

1) Proper fit, even a $1,000,000 bicycle that doesn't fit, is going to yield poor performance.
2) Proper fuel, if you think the 4 food groups are Beer, Pizza, Coffee and Doughnuts then, even with a $1,000,000 bicycle, your going to get poor performance.
3) Proper training, if your technique is improper, then your performance is going to suck, even with a $1,000,000 bicycle, your also likely to get more injuries.

Best thing is, $1000 for a bicycle, a $200 fitting, and $400 each to the nutritionist and a former drill sergeant named coach, will yield better performance and more speed for $2000, then spending $10,000 on a bicycle.