I don't think it's a straight formula of $50 per 500 miles. I was offered PHL-LAS day-of-departure upgrades last year for $150 total.
Interesting - I didn't realize it had changed. Is there any chance the "discount" you were offered was specific to the destination (LAS)? If $150 is the new transcon price, I would be inclined to pay up on occasion in lieu of miles - especially when the cash+miles upgrades come into play.
If the upgrade PHL-BGI is offered for only $150 each way, were I flying on a special occasion I think I'd be inclined to pay for it - at least in one direction.