It depends on the price of said fusion stones. But assuming you buy the ones that are, for the sake of example, 2000 gold and assuming you're using t3 cloth belts from Obsidian (the cheapest t3 you can get from a vendor, I don't remember offhand but I'll estimate 1k each), divide 50m by 3k and you get 16666 t3 stones; 5555 t4 stones, 1515 t5 stones, 505 t6 stones, 168 t7 stones, 56 t8 stones, 18 t9 stones, 6 t10 stones, 2 t11 stones (plus 1-2 leftovers on almost every tier). If you took it all the way, you'd end up with 2 (very dirty) t11 stones, which are useless on their own, so if you wanted a useful product, stop at the t10s. Or you can look at it in reverse. 3x t10 -> 9x t9 -> 27x t8 -> 81x t7 -> 243x t6 -> 729x t5 -> 2187x t4 -> 6561x t3 -> @3k each, 19,683,000 it would take about 19.5M gold to make 3 t10 (very dirty) stones. It would take about 59M gold to make 3 t11 (very dirty) stones.
On the other hand, clean t3 stones are about 80-150k on AH. 28M would get you enough to make a clean t7 from a t3. Then all you need are your mods. If you can get the drops to mod, you only have to worry about puris, if not, you'll have to buy from AH. I'll let you in on a secret, you can sell 20,000 gold on AH for 4 dias. Puris on sale cost about 29 dias or 25 dias each if you catch the 8x bundle. If you have 6 items you want to make in to stat stones, you'll need 6 puris. That's 174 dias. 174 / 4 = 43.5. 44 x 20,000 gold = 880,000. Sell a total of 880,000 gold in chunks of 20,000 for 4 dias each (total 44 auctions) and you've got enough dias for 6 puris with 2 leftover for whatever. and 20,000 gold for 4 dias will FLY out of AH; 2-5 days will get you your dias, then just wait for puris to go on special and buy all you need. How much would 880,000 gold get you if you were buying straight stones with the stats you want? 2 maybe 3 stones. For the better ones, 880k won't even buy you 1 stone.