If you got a bike where most of the bits on it were ok for less than £150, then bought a new rear wheel for around £50, and it had decent tyres, BB, no major spacing issues and the front wheel wasn't buckled or the fork weren't bent, then it would be not too hard to build it under £200.
But most of the bikes that come into my shop are coming in with one of the above problems.
If you got a bike where most of the bits on it were ok for less than £150, then bought a new rear wheel for around £50, and it had decent tyres, BB, no major spacing issues and the front wheel wasn't buckled or the fork weren't bent, then it would be not too hard to build it under £200.
But most of the bikes that come into my shop are coming in with one of the above problems.