Nobody out there that builds sumps for people is going to be able to do it cheaper than a mass-produced model. It's like trying to save money on a new car by having it hand built.
You wouldn't want to have just any Joe-Blow do it either, you want a "pro" because a sump is something you can't afford to have fail on you.
I've built sumps and wet/dry filters before, and as expensive as they seem, it's surprising how expensive they are to build. The last sump I built I spent something like 140 bucks in materials, mimicing a design for a sump that sold for 170 or so online. That was for me personally; I'd have charged at least a couple hundred dollars for labor above that number if I was building it for someone else.
If you're really determined, there is one site I know online that makes custom sumps. I'll probably get my hand slapped for this, but google the word "melev" and you may find what you want.
Cartman makes a good point. If money is your only driver behind this, you can do it really cheap using old tanks, rubbermaid tubs, etc. I still have an old AGA 10 "sump" on one of my reefs.
