I have almost finished a 3 frame obs hive. I found a number of plans for obs hives and checked them out. In the end I did not follow a set plan but worked out the correct spacing (paying attention to advice in Michael Bush's books). I spent quite some time drawing it out on graph paper.
Getting the uprights rebated and slotted for the glass and routed for the frame slots is the trickiest bit. I used a couple of pieces of 90x45 dressed pine and used a table saw (carefully adjusting the height of the blade to get the right rebates).
The most expensive parts were the glass, I purchased 2 sheets of safety glass which cost $110.
If you have a bit of time and a table saw you can build one pretty easily. I'm not very skilled at woodworking, but this project is not too complicated.