Figure about $700 per axle if you have 4x4.
But, a change from 3.55 to 3.73 is not very much and I'm sure you are just looking at the difference in "tow ratings" between the 2 gear sets...
Yes, the 3.73's give a better 'tow rating', but you still have the same payload and other weight limits on the truck, so it's not going to be a proportional kind of thing all around.. ;)
The actual difference is only a couple hundred rpms in direct or even OD between the 2. I had 3.55's on my 97 F150 and thought about changing to 3.73's, and while it would have been 'better', I would have had to do 2 axles (4x4) and the cost just did not justify it for me..
If you are really looking to be able to use different trans gears while towing for example, being able to hold OD better, or being able to stay in 3rd instead of shifting to 2nd on the smallest of grades, going to 4.10 would be a bigger change that you'll actually be able to feel and use..
That's just my opinion on the deal, and there are some that have done the 3.55 to 3.73 change I'm sure that are happy too...
But, like I said, if it's just to get to that magical "tow rating" figure... I'd not waste the money.. ;)
Good luck!
Mitch