Given how hard it is for a level 9 pc to gain XP at the moment, 6% of their current XP is enough of a tax I think to make sure that noone would ever cast this spell more than once.
To be clear, that means raising a player who is precisely level 8 would take away about 1,680 xp from each player. That's 18.6% of the xp required to go from 9 to 10.
A much smaller flat rate would be better, but I don't personally think any XP tax on players over level 8 right now is worth looking into given how hard it seems to be to gain xp in the current module. I don't think its fair to players to put such an emphasis on slowing down XP gain at high levels and ALSO tack on an extra tax on certain spells.
I'd prefer to see more laterally applied taxes to spells like this. Alignment shifts for raising players outside of the 1-Step range. A more significant and ritualistic casting procedure. I'm sure we can come up with something more interesting than just penalizing players that are already being penalized.
As far as quick fixes go- Having the crafting option available (In Fine/Enchanted Diamonds) as an alternative is a good, but it shouldn't be the only option. Making reagents available to find in the module or able to be purchased for a decent sum would also be required. You shouldn't HAVE to be crafting or have an available gemcrafter online in order to cast the spell, it should just be preferred.