I generally agree with the previous replys. However, if you can't get a fair deal on the used market you might as well try to see if it will work for your application.
What are the tank edges currenly sealed with?
How thick is the glass?
What are the dimensions of the tank?
What is the minumum depth you can get away with?
Does the species in question require a land area? If so you might be able to silicone in a glass ramp that will reduce the total volume of water in the tank, staying within the limitations of the construction.
My thought here is that if you can get away with 4" of water or so and put in a ramp/land area, the glass may be more than strong enough to hold that much water. It won't be very much.
You might post this on a fish forum, specifically one were there are a lot of experts on water volume, depth, and tank requirements. Most of the limitations listed by manufacturers are very conservative. It might very well be that this tank will hold 3" or 4" of water quite well.
Now if this is for a turtle species that will eventually outgrow a 45 gallon tank then it might not be worth it. But if it's for a species that will do find in a tank that size (maybe Spotted Turtles??) for a lifetime, then it might be worth giving it a shot.
Answer the above questions and I'll send an e-mail to one of my fish friends with a fairly extensive background in tank construction. He may have some input.