It would be silly to buy a bike with the intention of converting it, unless you really loved a particular bike but wanted it to have only one gear. (I'd never ride one, but imagine a carbon fiber Scott with White Industries eccentric hub,, carbon rigid fork, carbon bars, etc. maybe 17lbs...) Flat Fender is close, but you could use even less -
shorter chainring bolts (use the stock 32 chainring)
(use rear derailleur as a chain tensioner)
Following on what Thor said...what bike specifically did you have in mind? Maybe some of the old hands on these forums could point you towards an SS specific (or frame and parts build) that is similar in performance/price, without the need for converting as soon as you buy...
If you want to go ultra-cheap you might find a bike at a Pawn-shop and convert it using the methods people previously mentioned. Aim for old Treks, Specialized etc.
If you want a new bike you have lots of options. Many companies are builiding affordable single-speeds like Redline, GT, Haro, Cannondale, etc. If money's no object then I'd go for a bike like you ended up with at Webcyclery.
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