My reasoning is that enderpearls are extermely hard to get without being able to visit The End. Also they should be cheaper to buy because non flying members need them to transport around larger islands quickly. Also, $300 isn't OP because of how rare enderpearls are.
If it were this cheap it would defeat the purpose of challenges. Challenges are meant to be, well, a challenge and it certainly isn't that if you can just buy everything from the shop.
Endermen are pretty easy to kill actually. They're spawn rates up drastically considering there is very few places for them to spawn. Just make a large platform, and you will find an enderman every 5 minutes. By the way, what are the rates at currently?
Sorry, from your wording it seemed as if you were asking for it to go down, specifically your points about it "not being OP", that sounds like an excuse when you want the price lowered and sort of sounds like you want them to be cheaper? Although 300 = 1 1/2 stacks of sugarcane which really is quite easy to get, on top of Reaction giving you almost 300 a win, so if it was going to increase I'd rather have it increased a lot more.
Yes exactly 1 enderman every 5 minutes = a stack in 40 minutes IF every enderman drops 2 pearls which is rare.
Again, taking the fun out of challenges. They're not supposed to be something you complete instantly, they're something you work towards. A stack in ~1 hour (assuming slightly more accurate drop rates) sounds fine for now and I believe End biome will be added soon, to make it even easier.