The Enterprise versions of Windows will never be in the free zone. The free upgrade is for consumers (and businesses running the consumer versions).
There are a lot of unknowns. Pricing, support structure, update frequency are all high on the list. It's looking like 10 may very well be a solid product. Redmond has to make it too good to say no. Corporate America proved that they will ride a horse until it has to be put down with XP. Win7 is a solid product and it only gained a majority due to necessity, but EoL is 2020 and that's five more years of not having to make a decision. Win10 has one advantage in the number of aging boxes that initially had XP.