Earning Diamond (15k TC) can be done in one day, or two days for the same cost.
You earn a bonus for 25k coin in, 5k TC bonus.
You earn a bonus for 50k coin in, 10k TC bonus.
The best EV situation is to grind out the DDB machine at the Rio, but you'd have to coin in 5 times as much as a regular machine, and your variance will be sky high. You'd coin in 250k, or 125k 2 times playing $50 a hand, which will be 5000 hands. There will be an extremely negligible loss, but your actual results may vary..... Your looking at a 90% return if you run bad on 4oaks, premium 4oaks, and don't hit a royal.
I prefer 0.25, or 0.50 10 play 7/5 bonus. The return is bad (98%) but you only have to play 50k coin in. Sometimes I play 9/5 jacks (98.5%) $5 if I need to coin in real quick. I did try $1 Double Super Times Pay triple play, and lost like $7k putting through 50k
Try to avoid high variance games, unless you can afford the loss.
I play for Seven Star every year and usually put in a 25k/50k session once a month on average. I am a local so I do get freeplay offers which offset the losses. Losing 2% on 500k coin in (lowest amount to play to get to SS) is about 10k. My offers average out to about $400-500 a month, so it's costing me significantly less then the expected loss. Also I travel a lot so the free rooms at Harrah's throughout the US helps.