I had mine clear powder coated for $AUD40 (about $US25), but I had to do all the prep and masking of threads. I'm pleased with the quality of the job so far.
I sanded mine by hand to 800 grit. If I were to do it again I would go further, maybe 1200 grit,and I would pay more attention to the areas around the lugs. Once the powder coat goes on, scratches tend to show up a bit more than before, so careful sanding and polishing are crucial. I would also pay more attention to the brazing, sanding it more to neaten it up a bit more and polish it to a high sheen. Polished brazing looks like a neat lug lining job, and the extra effort will be repaid.
I can't stress enough how important cleanliness is for a good job. The more the frame gets polished, the higher the likelihood that fingerprints will show, and finger prints will etch into the steel very quickly. I have however considered leaving a single thumbprint as a feature and for identification purposes.
It's worth shopping around for a powder coater who understands your requirements. It shouldn't be an expensive process..powder coaters use clear coat frequently and they should be able to include your frame with other jobs, without extra set up costs.
Also, if I do this again, I would have the frame abrasive blasted first. Mine was chemically stripped, and then they used some sort of powered wire brush which left fine scratches. Most of the polishing was necessary to get rid of those scratches.
IMO, clear powder coat will be the toughest and most durable finish available and give the most bang for the buck. A 2 pack finish is also likely to be pretty durable, but I have no info on its use on bikes. I do know it's used a lot in marine applications and is well regarded in that area. I'd stay away from a polyurethane finish. I tried using it as a rust preventative measure. Polyurethane doesn't stick well to a polished surface, it will peel, and when it does, the whole finish has to be stripped and redone. Spray lacquer can bo touched up, but will probably require dismantling the bike to do so.
Lastly, if I do this again, I may just sand the frame to 800 grit or so, and put some extra effort into sanding the lugs to 2000 grit then using a powered buff to bring up a mirror finish.
I'll be watching this thread to follow progress. I'm currently at the stage of assembly where only the cables and chain have to go on.
Pics WILL follow! Patience,patience...
Edited to add...Squirtdad's idea is a good one i.e. do a clear rattle can job to see how it looks (and to show up any scratches you may have missed) A rattlecan job will be reversible if you decide to powder coat later, but I wouldn't expect the same durability as powder coat.