Ok to minimize random guessing, lets put things into context.

1 galleon = 1 galleon

17 sickles in a galleon => 1 sickle = 0.059 galleons

29 knuts in a sickle => 1 knut = 0.0020 galleons

Silver unicorn horns = 21 galleons each

Wand = 7 galleons

Weasley vault in Gringotts = 1 galleon and some sickles

Fred and George life savings = 37 Galleons, 15 Sickles, 3 Knuts = 37.891 galleons

Triwizard Cup winnings = 1000 galleons

Omnoculars at a bargain = 10 galleons each

Harry said something along the lines of "Dumbledore won't spend hundreds of galleons on me" when they were discussing the firebolt

So we know it costs at least 200 galleons, and I'm willing to say it will cost less than 1000 galleons because it will probably be less than the prize money won from the Triwizard Cup, as that's a pretty big deal.

Now 200 to 1000 is a pretty big range. Let's consider these omnoculars. Harry bought 3 of them, which means he had 30 galleons at the time at the Quidditch World Cup. So it seems like 30 galleons is a reasonable amount to have for Harry at any given moment. Unless of course he took some more money than usual, as it is the World Cup, which is quite possible. Harry also told Ron and Hermione not to expect Christmas gifts for "ten years" when he bought them. I'm sure he was exaggerating, but it still goes to say, it seems that 10 galleons is quite a lot to spend.

Based on this, I'm going to say that the firebolt is at least less than 500 galleons. I'm venturing to say that it is somewhere in the 200 to 300 galleon range.