Tamus:
You're going to find that most of us started as you, made some mistakes then moved on...
When I completed my pond renovation in 03, I thought I had the greatest pond...900 gallons shaped like a B, 17' long x 6' wide at the top of B and 8' Wide at bottom of the B, 30" deep...with a bridge over the middle leading into a Japanese garden complete with 6' waterfall...
Then I found the NI board..
www.koi.mag.com and found out that my pond was marginal at best and that only 3 koi would fit into it comfortably 5 if I wanted to squeeze them in...When I put 3, 15" koi into it, it looked empty, added 5 more... Then they started to grow, at 20-27" pond got crowded so had to build a second pond...
My advice is this: find a thread started by Akai san, click on his name to get his profile, then go to "threads by Akai San" and click on that, you'll a lot of questions about building the proper pond from Concrete footings to Filtration...Take you time and read the responses...
Re: Estimated costs: For the average pond say 1-4k gallons, filtration will be the biggest cost factor at about US$2-3000...After that it's the labor and materials...If you can pour concrete and lay block you can cut 40% of the total pond cost...If you're handy, you can learn...
You'll have to decide on an above, in or a combo of above and in ground pond...I have an in ground Japanese garden pond and an above ground pond...Aesthetically the in ground pond looks better, however, for koi appreciation, an above ground pond is better, you can sit next to the pond wall, lean you arms and head on the wall go to nose to nose with the koi... A lot better than standing and looking at the koi 5-6 feet away...
Hope this helps, just remember, take your time and ask as many questions that come to you, there are no dumb questions...
Aloha! Mike