it randomizes into a different tower with a similar total price, including upgrades. This is why the dart monkey only randomized into glue, it was the only thing close enough.
The towers randomize into other towers based on their cost, within 20% more or less than the total cost, that's why the dart monkey always turns into a glue. Atleast that's what I know from watching ISAB play with this mod.
if my knowledge is correct the tower random generation is based off how expensive the upgrade you got is and it randomises into something close to that cost