I've got a Trek Elance 400T, and I was curious about doing a SS conversion. I like the simplicity of having a single gear, but I want to know more about it. What parts would I need, and what's a ballpark estimate of the cost?

Read Sheldon Brown's articles on fixed gear conversions, it will set you to the right track.
https://www.sheldonbrown.com/fixed-conversion.html

The cost depends on how generous you are but a simple conversion could be around $200 or so, could be more if you are planning to get a new set of wheels.

Would it make a difference if I were going for a single speed freewheel rather than a fixie?

It would be a lot cheaper, do you have a freewheel hub or a freehub hub?
Also, if you can salvage parts from your bike, then it shouldn't be really expensive. Post a picture of your bike maybe?

I have a freewheel hub. It is an older bike, so it's got a threaded freewheel instead of a cassette (I would know, I've replaced the freewheel before). As for the pics, I've got plenty. Here are a couple:

Perfect, you seem to know your stuff, all you need is a singlespeed freewheel, new set of chain ring bolts, maybe a new chain ring and chain.

However, you might need to redish your wheel. If you know how to redish your wheel, wonderful; if not, take it to a shop and let them do it.

You might need a different quick release for the back because I head that plastic ones aren't too great for single speeds? I could be mistaken.

Can I use my cranks with a single chainring, or would I have to keep the triple?

I think you would only need to remove two of the chainrings you don't need and get shorter bolts. You can still use the cranks.

^What he said.

You might need a new bottom bracket spindle if you can't get a good chainline.

Quick release is fine on a singlespeed, since it can coast.

This makes no sense.

If you're talking about the skewer nut, nothing important is made of plastic. Threads and serrated surface will be made of metal, but it's really the dropout-locknut interface that provides the grip, so all you really need is a skewer that can get tight enough (i.e. steel, closed cam). In any case, if it worked with the small ring on that triple it'll certainly be good enough for a fixed gear or a single speed.

Can you kindly elaborate?

There's no difference between the torque you can produce on a freewheeling vs. a fixed gear singlespeed. In either case, it's almost always less than what you can get out of the small ring on a triple. Ergo, proper quick releases work perfectly fine on fixed gears, so whether the bike can coast or not makes no difference at all.

*Edit: Sorry. It doesn't make no sense, just implies something of a myth.

Why is it that a SS conversion is easier than a fixie conversion?

I see...I stand corrected, thanks for pointing it out.

Where did you read that?

Maybe not easier, but it seems cheaper. You said for a fixie conversion it'd be around 200 bucks, but the parts list for the freewheel conversion didn't seem like they'd come anywhere near that price.

its not really, just more costly as you have to replace your backwheel so you have a hub threaded for a fixed cog and left hand thread lockring

Chainline's a little less critical, but unless you think you need to replace your rear hub with a track hub (you don't, really, unless you're skidding all over the place) it's exactly the same. Once everything's set up you can freely switch between a freewheel and a fixed cog if you want.

So is this the general process? Remove excess hardware (deraillers, etc.), take off the extra two chainrings and use the middle. After that, I remove the old freewheel, and respace the axle/redish the wheel. Then I take links off the chain to make it suitable? So this works for SS and fixie?

Pretty much all you need to do, if you are planning to use your existing chain, make sure to get a freewheel in 3/32".

What about this freewheel? https://www.performancebike.com/bikes...peed+freewheel It says it works with either. And what I'm concerned about is respacing the axle and redishing the wheel. Is that something I could do myself?

I can't see the website but I'm assuming that it is the Shimano freewheel on Performance? If so, it would work with your chain. Respacing is easy, you need cone wrenches to loosen the nuts and you tighten the nuts one side or the other. Redishing could be a pain in the ass if you don't know how, I think Sheldon teaches you how to do it but might be hard to do...taking it to a shop might be a good idea.

Ya, it's the shimano freewheel. In addition to Sheldon, I also have Zinn and the Art of Road Bike Maintenance to help me along with repairs.

Do I need to buy anything besides a new freewheel and chainring bolts?

Maybe a shop visit or two? If you've got a good tool box, I don't think need anything else.