I would guess around 10k-15k for the summer (though I personally haven't been involved in any), depending on amount of funding, etc.
See e.g. <a href="http://rethinkdb.com/jobs" rel="nofollow">http://rethinkdb.com/jobs</a>
No personal experience (currently applying for positions), but I know Skritter recently posted an offer of $4800 for 3 months plus housing and airfare.
<a href="http://blog.skritter.com/2010/04/summer-intern-wanted-software.html" rel="nofollow">http://blog.skritter.com/2010/04/summer-intern-wanted-softwa...</a>