I have a Univega Activa Action frame from which I want to build a fixed/single bike. I've stripped the frame as much as I can but will have to take it to the LBS to finish the stripping. And I'm thinking I'll have them build the bike. I have the handlebars but everything else will have to be bought. It take 700cc tires and I want to have 35 or 38's put on it because I intend this to be my winter bike. I don't want/need top of the line components, only good enough to get me riding. I see that I could have https://bostonfixedgear.com do a custom bike for $265. Think it will cost me more or less than this to have my LBS build my bike?

Best guesstimate? It's a hybrid.

It'll cost about the same, if your LBS agrees to do it, or more actually because they probably wouldn't use 'refurbished' cranks. At the end of the day, it's not really gonna be a track-bike-looking-thing. It'll be a converted-hybrid-looking-thing. Those things convert better into cyclocross-looking-things. But I'm just guess estimating. Post a pic.

hard to accurately estimate without pictures, i think. hard to say what can be reused on it's current setup.

Is it really a good candidate for conversion? Does it have horizontal dropouts? Does it have adequate tire clearance and fender eyelets for your winter ride? And what happened to all the existing parts you stripped off? Is there a reason you're only keeping the handlebars for the conversion?

$500 if you need to get all components (wheels, crank, cog, chain, bars, brakes)
$200-250 if you have a basically working bike and just want to fix the drivetrain.

I was curious and asked the forum this week - How Much Does a Conversion Cost. Check out the responses I got on actual conversion costs. Should help you calibrate both what the pieces you need cost and identify costs that you hadn't considered.

There are folks that claim that you can do it for $50 or some other low number - here is what I have seen:
1. Wheels are the biggest cost - around $160 or so for a flip-flop set seems common. Some folks jerry-rig their existing freewheel, but a fixed-gear wheelset is a real cost. (and don't forget the tires)
2. Cranks are second - some folks re-use their existing, which saves money
3. Bars are $30 or more
4. If you have a bike (not just a frame) you avoid the bottom bracket, headset, stem, seatpost, brakes, levers, and saddle, which helps
5. There are $30-50 in incidentals - new chain, tape, cables, housings etc.
6. Anything your LBS does is on top of this.
7. Some folks get cheap conversions because they "happen" to have all the parts in their basement already.