Reaper of Souls gem upgrade prices:
Marquise Gems - Drop freely.
Imperial Gems - 3 Marquise + 200k gold. (This level of gem occasionally drops in Master and above. For every one you find subtract 3 Marquise and 200k from the costs for higher level gems.)
Flawless Imperial Gems - 3 Imperial Gems + 1 Death's Breath + 300k gold (cumulative cost: 900k, 1 DB, 9 Marquise gems)
Royal Gems - 3 Imperial Gems + 1 Death's Breath + 400k (cumulative cost 3.1m, 3 DB, 27 Marquise, .)
Flawless Royal Gems - 3 Royal Gems + 500k + 1 DB (Cumulative cost: 9.8m, 81 Marquise, 9 Death's Breath.)
The highest level gem in RoS requires 9.8m and 81 Marquise gems, or a bit less if you find some Imperials. That's not a lot, especially compared to D3v's gem cost and quantities. Over the last couple of months of the RoS beta I accumulated several hundred of each type of Marquise gem, literally 300-500 of each, and picked up 1-2m gold per hour of playtime on Torment 1. There's no Auction House and gems are all soul bound so you've got to find them yourself, but the system is designed for an average player to be able to trick out their main character with half a dozen Flawless Royal gems after a few months of play time.
One additional detail; towards the end of the Beta the devs began moving gems into the crafting and enchanting costs. Lots of the better legendary/set crafting recipes required 1 Flawless Royal Gem, which was much too expensive in the opinions of every fan I talked to, and which the devs said was subject to change.
A more common and more reasonable expense was the price to enchant legendary/set jewelry. That was initially just gold and materials, but near the end of the beta it changed to cost one Flawless Imperial gem, + some materials - but no gold. That gem cost 900k and 9 Marquise to make, which shifts the economy long term, since many players would find Marquise gems, rather than gold the bottleneck to that enchant.