The mimosa hostilis is likely to yield around 1%, so 28 grams should equal around 280 mg. It can also yield as little as 0.5% or as much as 2%, depending on quality and extraction method used, so you may get as little as 140 mg or as much as 560 mg.
The only number I have on Desmanthus illinoensis is 0.34% DMT, so assuming that is a standard number 28 grams should equal around 95 mg. I have no experience extracting from D. illinoensis myself, but I want you to note that it apart from DMT also contains the inactive NMT and the toxic gramine that may be apparent in your end-product, depending on extraction method used.