From what I saw when I was pricing 21 & 24 hp tractors last summer, there was only about a $500 difference between the TC21 and TC24D. So maybe something changed!?!
As for differences between the TC24 and TC30, much of the differences (other than the obvious class/hp) are in updated designs, turning radius, gauges, convenience features, etc. The TC30 is a basic workhorse, available in gear or HST, it is an older design and well proven. The TC24 is the more modern with a load of convenience features. A more logical comparision would be the TC29 versus the TC30. The TC29 comes at a premium price, but gives greater hydraulic capacity, tighter turning radius, and a boatload of ergronomic design & comfort features not available on the TC30.
But don't mistake the TC30 for poorly made. It might be a basic machine, but it is plenty capable of doing hard work.