The way to think about it is that the better quality rifle you buy, the less parts you will need to replace.
For example. on a TM VSR-10. You can keep the triggerbox, replace the sears and still have a sturdy durable triggerbox. You can keep the cylinder, bolt handle, cylinder head and hopup rubber and upgrade everything else and have one hell of a rifle while keeping money in your pocket after upgrading. Sure you pay a bit more on the rifle itself. But you can keep more of the rifle you buy.
Its fine to buy a cheap APS/VSR clone if you intend on replacing everything internally to begin with, but if you plan to budget on a cheap gun, you're not going to get very far... Literally.