A basic sales rule common things need to be sold in volume, rare things don't. Common things sell fast, rare things don't. so how hard to do want to work at selling your fish? How many do you want to care for waiting to sell?
For a small hobby business, identify a nitch do it well.
GBR will get you more per fish but their care requires elevated levels and more expense and time.
Endlers you'll have to sell a lot more of but their cost to maintain and care are extremely low. I usually sell my Tigers for $8 per trio and $1.50 per fry but the cost of them is relatively low for the quantity.
I don't know how GBR care requires more expense, care, and time. I have had a breeding pair that I don't do anything to except water changes and feeding. I keep their tank a few degrees warmer than the rest, but overall care difference in negligible. Everyone talks about these fish as being super sensitive, but what does that even mean? I have had their tank fluctuate temps of about 3-4 degrees on a regular basis. Also, before the CO2 injection their pH was kept at about 7.6 (still breeding).
I say "Rams" I haven't kept any for years but everytime you go in the fish section here there will be a thread about Rams, BBR, Where can I get Rams, so... yo do the math! I didn't know they were sensitive at all when kept them, but I didn't try to breed them either.