Buying directly into C makes almost no sense here - 2 SWUs are not $5500. The two realistic options are:

1) Buy Y, use 2 SWU. This costs $1500. You gain roughly 12K miles for the flight. If you value SWUs at the "going rate" Coupon Connection estimate of 20K miles each, you spend 40K miles via SWU and your net cost is $1500 and 28K miles.

2) Spend 100K miles and some taxes and fees; I'll estimate $100.

The difference between the two options is 72,000 miles or $1400. That makes each of your additional miles worth about 2 cents. That's close to your cutoff, so I don't know what you should do. I would likely use SWUs, myself.

One significant factor here is if you have other uses for SWUs before they expire. If they were going to go to waste (or something small like ORD-IAD), they're worth fewer miles and the SWU looks more attractive. If they would instead go to something like ORD-PEK on new C, they'd be worth more and straight C looks better.