For me, I had never done anything concerning plumbing, mortar, concrete, blocks, etc. before, and I'm not strong
I was hesitating for a few days after I got my hole dug in the middle of my backyard. I was afraid I would not be able to do the footing, building the pond walls, etc. But then like you, I got some crazy quotes, one quote $4000 and another $8000 just to build the walls so I decided to do it myself and I'm almost finished only the cosmetics left.
I say don't worry, go for it. You will get many opinions, read them all, choose the ones that suits your situation best. It won't take you that long, one to two months the most. You'll find it to be not that hard
As far as the cost is concerned, I think the avarage is about 2000 to 5000 DIY. For me it's about $3000. You will have to buy the tools, but then you got to keep them or even you can sell them back on Ebay, Craiglists, etc after you've done
Yen