Well... "cheaply" is all a point of view.
You can either fix only what is broken with stock parts off ebay that you got cheaply, or you can try to make your rig better than stock and use some ebay parts when available to keep the bottom dollar down.
Usually the savage issues are diffs and center axles/cups.
For diffs, there are two ways I know of that help things last longer.
1) You can go CEN diffs, not sure of what all that entails, but it's basically taking some really beefy diffs and throwing them in, then dealing with a new gear ratio.
OR
2) I've had decent success (aside from the bearing blow out) running alloy diff cases with XL diffs (taken apart and replaced the outdrives for ones that fit in alloy cases) and properly shimmed ring/pinion mesh. The alloy cases don't allow flex you would get from plastic cases, so your gear mesh stays truer longer and can handle more pressure from a torquey engine without flexing.
You can get diff cases for around $35 a piece:
http://www.liquidrc.com/index.php?main_page=product_info&cPath=234494_129&products_id=45365
I upgraded/rebuilt both of my diffs last winter to avoid having diff issues this season. I bought the truck used (traded for a buggy I wasn't using much) and it was a general modge podge of newbizm's that I spent the winter clearing up. In doing so, I ditched the diffs and replaced them with XL, changed the housings to alloy (from the link above), shimmed them nice, used new bearings, put 50K oil in them and installed XL brakes. Along with a few other things...
Anyway, all of that was done to save money and aggravation in the future, i.e. now! So, really the only issue I've had was that bearing blow out... and center diff cups wearing out quickly. Hopefully FE's bearings hold up better than HPI's.
works though 
