Because stuff you make yourself is free, amirite?
Congratulations on your baby steps towards becoming a manu-bear. Having been down that road I'd recommend making a spreadsheet to help you evaluate selling intermediate goods and buying final product vs. doing end to end manufacturing yourself. You may be quite surprised, in a wallet-inflating sort of way.
The POS fuel being made from Planetary Interaction is new, and I don't expect the prices to stay this high once everything settles. Just like how I made a lot of ISK the first months of making medium rigs, but profits shrank quickly once the market stabilized.
I expect Coolant prices to come down just like rig prices did (if for no other reason than it is easy to make, so more people will realize this and start making and selling it until the profits are back down to Earth.)
I actually don't build the final product, I am just doing a complex reaction and selling it to people who will build the final product. At the prices 1 week ago, this would have net me 120M ISK a week. POS fuel prices have gone up 50% in the week since, and now the profit is far less (about 80M now). I expect this spike to come back down, but in the mean time I will make the most expensive part of the fuel myself (which is one of the easier fuels to make, surprisingly.)
You are right that this hobby will probably be a big money pit (it is starting to seem that way). Even so, I don't regret it. It is a learning experience, and something new for me to do. I hope it really does work out well, but if it doesn't it is no big loss.