The most affordable custom rifle is the one that is built exactly the way you want it without the concern of cost.
If you build several affordable rifles you'll end up with something you won't be quite happy with and then you'll sell them at a loss.
You want affordable, get a factory rifle, want a custom, get exactly what you want.
Yep I've gone this route and found it true. A rather expensive lesson I might add.
That having been said you should ask yourself what it is you want (what you're looking to accomplish that can't come from a factory) and then build with those questions in mind.
A Remington 700 build like you describe can't be bad but if you're looking to do a lightweight .280 Rem maybe a mountain rifle with a McMillan added can get you there for less money? Just an example but you see what I mean.
Higbean raises a good point too. If you want a really accurate rifle built to your specs that can be a good way to go.
Good luck! This is an expensive but rewarding addiction...
