Been there, done that. Although, I did it over years. I am a NAUI Technical Support Leader, and I have been diving for thirty-five years.
What Joel says is accurate, but as he says, you'll be hand held. You need several years of dives to be a lead diver or be ready to do "Big Boy" dives (doing the tough stuff on your own).
Wreck and/or cave penetrations are their own specialties, and require training applicable to each type of diving.
In addition to up front costs for training, you'll need to be diving consistanty, and that adds up.
To borrow from Joel (and these are minimum entry costs):
Dry Suit with hood and gloves: $2000
Masks, 4, $200
Fins, 4 sets, $300
Back plate and wing, 2, $1000
5 high performance regulators, $2000
1 suit inflation system, $200
2 mixed gas computer, $800
3 depth bottom timers, $200
2 compasses, $100
1 high peformance light, $1000
2 back up lights, $200
2 lift bag / SMBs, $300
1 primary reel, $50
1 secondary reel, $50
1 spool, $50
2 cutting devices, $50
3 sets of doubles, with manifolds, $1500
6 stage bottles, $600
2 suit inflation bottles, $200
2 slates, $80
1 oxygen / helium analyser, $100
box of assorted clips and dodads, $300
box of extra hoses, $400
gear bags and crates, $300
back up wet suit (5-7 mill), $300
Training:
Advanced Open Water, $250
Rescue, CPR-First Aid, $250
Nitrox / Advanced Nitrox, $350
Decompression Prodecures, $450
Gas Blender, Advanced Gas Blender, $350
Wreck Diving - External Survey, $400
Extended Range - Trimix $600
Advanced Trimix - $600
Totals: $14,530
Again, those costs just get you started, figure that you'll need to pay for dives as well - gasses, boat costs, Divemasters, insurance, additional equipment, yearly maintenance costs, etc.