As others have already said, DUPLO can do well but you have to factor in a few things:
1. The boxes are much larger than comparitively priced LEGO sets. This can result in larger shipping costs and requiring more storage space.
2. "Generic" DUPLOs fare far less well than some "Generic" LEGO pieces, so be extra wary of buying the Box O Duplos. For example, the LEGO Creative Tower is selling for 101.35 right now on Amazon, and the DUPLO tower just 55. Both could be had at the same price, of course. Now if you spent $30 on each of those, you'd still be making some money on the DUPLO tower, just far less than the LEGO one. Similarly, people are not likely to buy a themed DUPLO set for specific/rare pieces for use elsewhere.
3. Margins are definitely lower. Where as one can usually take a gamble at 20-30% off of a LEGO set, definitely look at 50% or more for DUPLO purchase - One exception, I have been purchasing the DUPLO Disney Princess sets at 20-30% off as I have found them, and some of the returns have been quite good.
4. Personally, I'd avoid the PLANES theme like the plague.
5. DUPLO sets seem to have a lower ceiling in general. While lunatic Star Wars fans will pay $700 for a Star Destroyer, in general it seems even the best and most expensive DUPLO sets top out in the $150-$200 range. And those are rare. Very few go higher than that.
6. DUPLO seem to have a longer product cycle in general, as they seem to require less updating. 2-3 years is pretty standard as opposed to the 1-2 years for most of the LEGO sets, which means if you get in too early you may have to wait a bit.
In your example, Big Farm and Train at 30% are probably fine investments in that you will profit from them, though there's no telling when those sets actually will retire, and that money could better be spent now on other sets that are already disappearing - Most Walmarts are also clearancing Starhoppers, Malibu Mansions, Fire Temples, etc etc which may result in faster returns with smaller storage commitment.
I personally think the Super Hero DUPLO theme will do quite well in the long run, and am definitely eyeing them. Most Target/Walmart are clearing out the Superman set now.
So there are plusses and minuses, depending on your strategy, it could be worth fitting in.