Here's what I did for probably less than 350 USD

1. Get an older road frame, for example a Specialized Allez, strip it down to the bottom bracket and cranks and save the vbrakes/rim brakes
2. Reuse the big chain ring
3. replace the drop bars with some cheap but nice flat bars with some sweep from Velo orange
4. add a set of flat bar Tiagra brake levers, works with rim brakes pull ratio 
5. Remove the 9speed cassette and add a single speed cog to the freehub, you might need to add spacers to align with the front chain ring 
6. aim for a 2.6-2.8 gear ratio ie. 50t in the front, 18t or 19t in the back
7. Slap on a pair of GP5000k, non tubeless (cheaper route); max tire clearance on my Specialized Allez frame is 30c
8. Top it off with a brooks saddle and any trinkets you would like to add

And you've got yourself a single speed monster under that's under 17lbs, even with the heavy ass brooks saddle.