About $45. You'd probably be better with a towncar service - I believe they are cheaper, but I don't know you'd be in your room much before 8:30 even with a cab. Check the transportation board.
You often have another 20 minutes after arrival time before you are off the plane. It takes another 10 to get through MCO - assuming you don't end up in a long line to the ladies room. No checked luggage, but you are now (assuming 7 pm arrival) at 7:30 and you haven't left the airport. Checked luggage adds another 10 - MCO has really fast baggage claim - and I suspect its even faster since the ME implementation limits the bags seeing the carrosel).
You catch a cab - its still half an hour from MCO to the Poly (if you catch a speedy cab driver, maybe 20 minutes. If you catch traffic, an hour). Arrival ETA 8:00. Avoiding ME might save you an added 20 minutes - maybe more if the Poly is a late stop in the ME rotation - or it might not save you much of anything - we've had trips where ME has added an hour (waited for the bus to leave, last stop on a four hotel route), and trips were ME has been really quick - immediate check through, immediate boarding, immediate departure to the resort, which was the first stop).
You get in the check in line at the Poly. For some reason, my own Disney check in experiences are always slow. And once you check in, since you have carryon luggage to drop in the room, and 8:30 in your room time is pretty reasonable with a 7:00 arrival time at the airport - even without ME.
I'd plan on Captain Cooks. Or, if you have energy, hopping over to DTD. We never have energy for anything other than the resort we are in.