From what I've seen passing through Texas and Ca. oil drilling areas that doesn't seem like much of a job, take down the pumps and cover the holes and you're done. Or did I miss something.*
* I passed through these areas, you work in the industry. If I'm wrong, please set me straight. 
Oh, you have to pull the rods, if any and tubing. Set plugs, pump cement. Then you cut the casing off below plow depth...greater than 3' anyway. Cost is about $60k to $80k per well for those gas wells from 5,000 to 8,000 feet deep around Farmington and Durango. That includes money to remove all of the equipment, clean up everything and reslope and plant the area where the well was.
Not bad...the worst thing is getting the permits to abandon and the overall government relations...then of course the government must approve your procedure...then every now and then you get some jerk like Michael ran into on the shooting range and they try to layer a couple of hundred thousand dollars in improvements to their area on you until you get a lawyer and tell them to pound sand. Mostly the government guys are really good to work with....it's just when they send out some starry eyed idiot from DC to straighten things out the real problems start....then you have to acquaint them with contract law and criminal law to show them how exposed they are to damages for being stupid. Asked an MMS representative once to confirm he was instructing me to perform a felony to avoid issuance of a fine and so long as his legal department delivered the demand I was alright with it...well, that one went away pretty quick....
Offshore, cost is about $180k to $300k per well, if on a platform with other wells in shallow water. Costs offshore can easily top $1,000,000 and that doesn't include platform removal which can be several million more. I had to cut the wells off 16 feet below the mudline. If you have to mob out for a single well that can't be abandoned without a rig...you can spend that million plus some.