Map packs don't require as many resources to create. You can use its sales to judge the general interest in DLC.
But that is just it. You Can't. Its not that you 'may' not, but it is simply impossible to gauge DLC interest on a type of DLC that people do not want. Generally, maps are just added to the pot.
I agree that something like, say, a "faction" DLC may require too many resources .. but as people say, units, heroes, monsters, spells ... THAT is where the real content is here.
If you want to release a map-pack to gauge interest, fine. But realize that the results will be negatively skewed.
And this is from someone that has only played on pre-made maps thus far
If I were to sell a map-pack, I'd probably waver between $0.99 and $1.99.
If I were you, I think I'd focus on adding a few heroes (simple enough to make), make a new monster or two (a bit harder), maybe a weapon or two (somewhere in between), and a few cool maps
Maybe a lesser version of that, like, say, 5 heroes, 1 monster, 2-3 new 'loot items', 2 maps.
Then gauge a price on that accordingly ... but I really don't think you will get good results with a 5 dollar map-pack, if all you are selling is maps. I mean, BY ALL MEANS sell maps expensively if you want, but DO NOT use it as a way to gauge the market for DLC.