The only real part cheaper on a buggy vs truggy is the tires. Don't let that be the biggest thing to sway your opinion one way or another. But i will say this, if you intend on getting into both truggy and buggy, buy the same brand. Alot of parts are shared between them and this will save you alot of money. Also don't be afraid of buying a roller and completing it yourself, often times its the best route as you won't be paying for an undersized/underperforming engine/servos and etc. You want a good engine (not saying you have to spend $300 on an engine either) and good servos. Those parts are the most important followed by radio gear.
VP-Pro makes great tires that are fairly cheap in comparision to aka and pro-line, Ace/XP makes good servos that are powerful, quick and cheap. Go Engines make damn good mills with lots of bottom and top end power.