Tempered glass does not make a tank more or less expensive. Tempered glass isn't difficult to make, it's just safer because it's strong (which is why they use it for bottoms) and when it breakes, it doesn't crack, it shatters. "Safety glass", like they use for windshields and such, is tempered glass. When it breaks it breaks into lots and lots of tiny squares of glass.
I did a search for "top fin 55g tempered" on Google, and this is what I came up with from a reef keeping site (and believe me, they know their stuff!):
You can click on the chart to go to the thread I pulled it from. As you can see, the person that posted the chart highlighted in yellow the 55g spot where it says whether the panes are tempered or not, and it says the 55g has ALL tempered panes.
I think a little more research to be 100% sure that the tank is or is NOT tempered would be a really good idea. For example, you could call the company and find out. If you can't easily find the number, take a trip to the store and get it off some packaging. IF the tank IS tempered, the SECOND that drill bit starts cutting, you will be SHOWERED in glass fragments as the bottom shatters. MOST tanks over a certain size that have been manufactured in recent years have tempered bottoms. The higher the volume, the more likely it is to be tempered. I can see where a 55g might have all tempered panes, due the the length of the tank and how narrow it is. Mine has a broken middle support and I've been told that the glass bows 3 inches or so away from the support when it is full of water (by the lady I got it from). Good thing I don't plan to FILL it with water.
Decided to take a bit more of a look around for you before I hit post and came across this:
How to tell if glass is tempered. - Saltcity Reef Forums. If you click the link, there are directions on using an LCD scree and polarized sunglasses to see if the glass is tempered or not. Hope this helps!