Here's kind of a no brainer that'll make you go "ohhhh....damn," after you read it.
Every day, you can get 1 Jewelcrafting token. This is good for 1 Chimera's Eye, or 1/3rd of a Jewelcrafting Cut.
If you take the Chimera's Eye, you'll get an instant bonus of 350g, every day (until prices start to drop, but just bear with me).
If you save for three days, the opportunity cost would be 1050g: the gold you would have earned from selling the Chimera's Eyes. I'm not sure about your realm, but on mine the cut rare gems go for around 100g, whereas the uncut go for around 50g. Some sell for more, and some sell for less, but on average it's about a 50g profit per gem.
Say you sell popular cuts, like the Bold or Brilliant Inferno Ruby, Timeless Demonseye, Jagged Dream Emerald, etc. and can sell 2 a day on a lower population server. After purchasing a recipe, it'll take the sale and profit of 21 gems, or 10.5 days, to make the same amount of gold that you would be making by purchasing a Chimera's Eye. This is a fairly long time.
Of course, YOU GET TO KEEP THE RECIPE AFTER THE 21 GEMS SOLD, so every gem sold after that is even more profit than if you would have bought the Chimera's Eye. /discussion.
Let's talk a bit about the Fire Prisms, as well. I've made a few, and they're not bad. HOWEVER, we're looking at the opportunity cost here. For a quick refresher, let's say that you mill x amount of ores. The cost of making the Fire Prism is whatever you paid for the orbs, we'll say 100g, after the excess uncommon gems are sold, and the rare gems are cut and sold (again, we're just using pretend numbers). The cost of the Fire Prism, therefore, is 100g. The OPPORTUNITY cost, however is 100g PLUS whatever you could have sold those 3 uncommon gems for. Again, your mileage may vary, but on my realm Nightstones go for 30g. That's 90g for the Nightstones, around 30g for the Zephyrite, 70g for Jasper, 45g for Carnelians, and we've covered 4 of the 6 gem types. 90 + 30 + 70 + 45 = 240g, it adds up quick. Hessonite goes for around 40g each on a good day, and Alicite is a bit cheaper at around 20g each. 120 + 60 + 240 = 420g.
On average, you'll get 2 rare gems from the Fire Prism. Cut, we'll say those are worth about 150g. So, 300g down off of the opportunity cost, and we're left with 120g. The chance of getting a Chimera's Eye, according to the WoWhead drop rate, is 9%, so we'll round up to 10% and subtract another 35g (10% of 350g), and we're left with 85g. So, we can see that if you sell each individual gem, you'll make on average 85g MORE than if you made the fire prism. Of course, if you're a lucky person and got a Chimera's Eye every 3 days instead of once every 10 days, then it'd be worth it (350g* 1/3 = 115g), but most of us will have to adhere to the 9% average drop rate.
-Purchase gem cut patterns over Chimera's Eyes - it will eventually be more profitable
-Unless you can find the uncommon gems very cheap, do not make Fire Prisms