I don't think you'll find a shop with stocked diamonds for 61 rupees, I think buying at 69r per diamond sounds about right to me.
i have close to an sc if not more. not quite sure if i am going to sell yet or not ill let you know if i decide to. reason im not sure yet is because i may use for part of a build im working on atm
wait it's 50-70's? jeeze... i bought a few stacks of blocks for 75r each per diamond lel (that was a few days ago)
I'm buying diamonds at 18721 for 74/each. Poked around all the servers with /v +shop & mall and have found no bulk sales with diamonds for less then that. Good luck with your search
I sell diamonds but if you want them for 74r or less it's quite a long waiting list. Since you have 50k, if you're willing to pay 80r each, you could get 625 immediately (9 stacks + 49). Or you can get 666 in about a week at 75r each (10 stacks + 16). Here's my thread if you want to place an order: http://empireminecraft.com/threads/selling-diamonds.43323