You said they usually start off ok but then come loose after riding for a little bit. This would suggest there is a little bit of play in the cranks and that's making the bolts loosen. If you use ptfe it'll help keep the bolts tight and reduce the play so you'll get a bit longer out of the cranks until you can afford new ones.In other words it'll stop them being a pain in the arse.
I was quoted around 300 from jaf for something similar to Damon's but different geo. Frame material, paint work and other extras all alter the price though. I ended up skinting myself though so I can't get it now.
Most people will choose their own bike because we buy the parts we like to build bikes that we like. I'm gonna put my ghetto bike up when I get round to taking some pictures.