The consistent quotes I've gotten here has been 5K including deep driven pilings with 10K lbs capacity. $10-12K in my mind is outrageous. continue shopping and consider doing it yourself minus pilings and electrical hookups...

I purchased a Magnum "S" boat lift from Boat Lifts Unlimited. I agree the Hi-tides are the best but you pay for them. I spent 3500 for a 8000lb lift with remote. I installed it myself. I added the extra long cables because of where I live on the Shrewsbury River (actually Branchport Creek). If I upgrade the cables to the next size it's good to 10000lbs. It is the best - works great with no probelms. Their support is great too. Bought it direct from them. They are based in MD.

Sold the Shooter, I now keep the 302 on it with no mods.

another closeup...

boat lifts
 

Bought a used 12000 lb. ,4 poster, that I saw while installed at original location for $2000. Cost another $2000 to have it moved to my site,purchase of (4)- 12" pilings and have them driven.It also incuded setting and assembly. Ran into trouble in Fl. hitting coral which can cost more.
(1) carefully inspect all steel work(if steel),pulleys,cables,drives,controllers etc. KNOW what you are buying.Remember it is used. There may be additional unforseen costs.Now is time to rehab, not after installed.
(2)Know what "extas" are with piling company
(3) Our area no longer "permiting" installation of used lifts.Check with your building code dept.
Had estimates of $7500-10500 for a 10000 lb.