Here are some tips for building custom Heroica maps. In no particular order:
1. Buying more boxes of the official games is still the cheapest way to get parts for custom boards.
2. Only use Bricklink for parts you absolutely need. Bricklink can be expansive and addictive.
3. Use Lego Digital Designer to design your map if at all possible. It does not have every conceivable part imaginable, but it will give you a good reference.
4. Inventory your Lego pieces. This will help you not waste too much time looking for parts for your custom board.
5. Custom boards actually do work best when the typical pieces used are like those found in the official games. They also work well making them modular in the sense that some of the 6×6 plates should be identical. Mainly using 8×8 plates, 6×6 plates, 4×4 plates, 2×6 plates, and 2×2 plates, and of course, those 2×2 modified plates with the stud really makes the board come together and be more easily re-configurable later. Also, 2×4 plates to connect the larger plates does not work very well at all. As noted above, buying more copies of the game is still the cheapest way to obtain these core items. That said, break the rules of the official layout schemes to your advantage and to achieve your vision. I use a lot of 6×8, 6×14, 6×10, and 6×12 types of plates as well.
6. Don’t over decorate your board. Perhaps a lot of detail makes them look better, but it affects playability in a negative way.
7. Don’t make a particular map too big. I am constantly scaling back my own boards (I’ll upload the new pics when I am done revising them) in both size and detail. This both saves on parts and allows more parts to be used for other maps.
8. Look to other types of official LEGO sets for usable parts as well. This too can be cheaper than Bricklink or ebay.
9. Use Bricklink and eBay. It seems like I have been saying avoid it. This is not what I mean. But what I do mean is that you should only use Bricklink or eBay when absolutely necessary. Getting Microfigs off Bricklink or eBay is a cheapest way of getting non-Heroica microfigs for your custom maps, and for getting those few select pieces for your idea that would cost way less than buying an official Lego set to get.
10. Be inspired by the work, rules, and ideas of others. My own MOCs are a mix of originality and straight up ripping off ideas. That isn’t a bad thing. Imitation is a form of flattery. Much of my board will not be there if I had no inspiration. I actually like playing or creating rules and potions and stuff like that more than building maps. Besides, most types of locations will be common anyway, and getting yourself going with custom boards from the inspiration of others can lead to complete originality later, as it did with me. Be sure to give thanks to your muses though.