Sort of a strange breakdown as the longer she's with you the more per hour it costs, unless she spends the night where it actually gets pretty economical.
2 hours @ $150 equates easily enough to $75/hr. But let's dig a little deeper at her other offerings...
3 hours @ $250 is $83.33/hr
4 hours @ 350 is $87.50/hr
But here's where it gets VERY interesting. Assuming "all night" is a 12 hour window - say 8 PM to 8 AM...
12 hours @ $400 is a very appealing $33.33/hr
Looking at this from a "best bang for the buck" (literally) I'd stay away from the 3 hour and 4 hour options. Technically, you can just book her services twice in one day at the 2 hour rate and save yourself $50. Maybe a nice noon to 2 PM session - then go watch a ball game with your buddies and maybe grab a bite to eat - then pick her up again for the 8pm to 10pm slot.