There is no limit on spending greens. Few seasons ago I used to spend over 100 packs per day and managed to get few 7* in short time. Problem isn't how many packs are you going to spend, probel is that you can't knonw if your player will be in good form or not. Unless you don't have super strong subs, how to substitute/replace 7-9* player which isn't in form? Would you sell that player (who costed you at least 100 packs)? I hope I've managed to explain it...