The price of fish within the same group can vary wildly due to wholesalers supplying from a number of different farms. Ottocinclus can be bought from either side of the world, with difference in quality, loss rates, carriage etc. While it's true shops can set their own price, many follow similar margins, so the variation you see is likely down to the aforementioned factors.
You can blame eye-watering business rates as one of the reasons for the price of fish in this country. Trust me, very few people get rich in the UK aquatics industry, most of the time it's done for love; even some at the top of the tree of big retailers are avid fishkeepers.
Support your best local shop, and maybe they'll have the opportunity to be more competitive, rather than running to online retailers for everything to save a quid here or there. I'd sooner hand pick Apistogramma who have paired up in the corner of a shop tank, than rely on some bloke in a warehouse in Milton Keynes to pick my fish for me and stick them in the post. Not to mention the invaluable advice a decent shopkeeper can give you.