Two-week sprints are exactly that, sprints that last for two weeks, they are not fixed on the features that are planned.
Those are targets, but if something isn't finished, it moves into the backlog and can be allocated to the next sprint, so fixed prices for two-week sprints shouldn't be a problem.
The rates you're mentioning would equate to $240-480k/year (based on taking 4 weeks off), if you're in Silicon Valley, this may well be reasonable, but in a lot of the rest of the world it would be a bit on the high side.
In the UK top-end contracts seem to top out at around £600/day, or £144k/year, about $190k US.
For a ballpark idea if the rate is reasonable, consider the salary you would otherwise be getting if you took a regular job, consider what multiple you're looking at (say you could earn $80k with a regular job, and you're asking for $240k, that would be a 3x multiple, which would be reasonable), if you're proposed multiple is over 4x, you may be aiming a little high.
Saying all that, your best bet may be to have a good idea what the minimum you'll accept is, and meet face to face and discuss, get an idea what kind of budget they have, and you never know, you may be able to get the rate you want, or more, if you're very lucky.