That’s a tough question. Usually, low-res pictures are termed that way by the photographer to give the impression that they won’t be able to produce quality prints out of. Instead, low-resolution usually find their way onto websites.
Bare with me while I get a bit technical here…
A quality print is usually around 300 PPI/DPI. This means that in each inch, there are 300 tiny dots, or points of ink that make up the picture. Obviously, the higher the DPI, the smoother and better the prints will look.
Typically in a magazine you will find at least 240 dpi, most likely 300 dpi, but some are using lower budgets and print with 150 dpi. Most pictures on a website however, are 72 dpi.
There is no way in knowing without more detail what size you will get, exactly, but usually low-res stands for something around 72 dpi, which is fine for e-mail, posting to the web.
There are other factors involved as well however, because the overall size of the image may vary as well. It’s possible to have an 8×10 printed at 72dpi, but it will look extremely poor.
I think that upgrading to High-res files is something that is important if you plan to do your own prints. Low res files are usually offered to people who are also ordering prints, and won’t necessarily be needing to make their own prints from them.
The photographer’s industry used to be structured on print sales, but now that digital is so accessible and people are becoming more tech savvy, selling the high-res versions of the pictures is the only way the photographer can survive. While it may be shocking to pay $1,500 and no be getting full-res digital files, the costs that are invovled in operating a business like this more than offset that cost, so charging more to sell the high-res files is a necessity if the client (you, in this case) is going to be using them to make prints and distribute them.
Hopefully that answers a bit of your question. Overall though, unless you’d have to sell an organ to afford the extra $500, I’d professionally suggest you go ahead and upgrade and buy the high-res, because anything above a wallet print in a low-res file will look horrible, in most cases.