Well lets take a look at this based on recent Ebay auctions. Those prices will fluctuate, but relative to each other should stay roughly the same until the next set is released.
SP Engine
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4x Cyrus Conspiracy - $20
4x Poke Turn - $25
4x Energy Gain - $10
4x SP Radar - $8
4x Power Spray - $5
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Total: $68
Now, I know that you wouldn't actually run 4 of everything in an SP deck, but you would want a full set just in case. This doesn't take into consideration the pokemon that you'd need to run. So we'll just grab a couple of prices of the main ones played.
Luxray GL - $5
Luxray GL Lv. X - $60 (To get one immediately or consistently it will be a bit more then this, but we'll lowball it)
Garchomp C - $5
Garchomp C Lv X - $5
Dialga G - $2 (Most leagues will have this as a free promo still.)
Dialga G Lv X - $25
Promocroak - $15
Uxie Lv X - $70 (Honestly is used in most decks, but almost required in SP decks.)
Verdict: A cheap SP deck, settling for alternatives and making creative choices will still run almost $150. LuxChomp is in the $350 range...
In my opinion, the best thing to do is buy a box or two of boosters then choose something you'd be interested in playing and trading whatever you're not going to use for things you are. That's how I've gotten back into the game. If you're looking to just buy a deck and be done, Pokemon isn't necessarily the game for you. ^_^