In general terms I think it is a good idea for you to establish a fixed day rate that you can use to easily respond to this kind of inquiry. Settling on the number can be tricky, it requires a somewhat objective judgement of what your skills are, your level of experience, and what your time is worth. You don't want to price yourself out of gigs, but you don't want to leave money on the table.
In a professional environment, something to consider is what are you overhead/social costs. A good rule of thumb IMO is to add 25-30% to your determined hourly rate for social costs (taxes, healthcare, etc.), then use that as your fixed day rate. I also encourage you to base your day rate off of a 10hr/day, as day-rate work is almost never a flat 8 hours. If you base your rate off of an 8hr day you're likely to wind up working an hour or two for free most days.
If you are going with $25/hr (arbitrary, taken from SMMeade's post - you will need to determine where to benchmark your hourly rate) then I'd say something like:
$25/hr * 10hr = $250
*30% Social = $75
Flat 10 hr rate = $325/hr
I'd recommend including these lines on your quote as well:
After 10hr mark, hourly rate of XXX applies (you need to determine that as well).
Travel expenses at cost + 10% administrative (save your receipts).
Also think about your payment terms, suggest those on your quote.
If you trust them, maybe you do 100% NET 10 (100% of payment is due within 10 days of invoice following event).
If you don't trust them, don't think they have the money, etc.. maybe you suggest 50% deposit, 50% NET 10 (50% of payment due before event, 50% within 10 days from invoice) - most companies won't agree to this but it is something to consider if they seem less than reputable.
