charles3526
Hello & Welcome to the Forum!

What a wonderful gift for your sons. 
When you say "collector"  set do you mean, pre war, post war, or modern?
O scale or Standard Guage? $800, should purchase a nice "collectors" set to display and leave money to purchase some inexpenive trains for your son's to play with.

I have O scale trains that turned 100 years old last year, they're worth more than when they were originally sold and still operating after 100 years of use.  Collectors value? I guess that would be a debateable question..
No reproduction train is ever going to be worth the originals value.  Trains are NOT an investment to be counted on in the future.  They are and always will be worth what the public is willing to pay for old/used trains,  Greenburgs is a good guide, but it is just that, a guide. It may or may not reflect the true value that people on the street are willing to pay for an item.  Look at the online auction sites and view their completed/sold listing to see the wide range of prices people have paid for identical  trains.

Please, don't think I am trying to discourage you from purchasing a "collector" set.
If you do, remember to spend a little for some trains for your son's to play with.

Happy Rails!
dave