[thejeff]

No. It's a discount. 80 gems to cast.
If you wish for gems, you get 25 of each gem type. In theory, you alchemize them all to pearls. 25*6 = 150/2 = 75 + 25 pearls = 100 pearls for which you paid 80. You take 80 of those and cast it again, leaving 20 to spend or invest.
In practice, you'll alchemize more of some type and less of others and/or use some of your S income, all depending on what you need for other uses.

Would it make more sense to think of wishing for gems as giving you 100 pearls? Except better, because you don't have to alchemize them back to pearls. Since you can, you can get 100 pearls by spending 80. Or 80 pearls and 40 other gems.

It's not "I cast wish at this site to get a whole bunch of gems and free wishes." It's "I cast wish at this site and get enough gems for another wish and a bunch extra."

The only real problem is that Wish comes along fairly late in the game and you might not have enough time to get a good return on your investment in building wish casters.