That is the 1957 PA/PA Northern Pacific set and is very desirable. The 490 is a dual motor unit, the 492 is unpowered and contains the diesel roar unit and the diesel horn. The value will vary widely with condition, if the roar and horn are operable, if the horn controller is included and if the powered chassis runs as new with no bushing wear. The OB's with a legible number stamp will add $50 to $100 to the value.
If you are not familiar with Gilbert diesels and how to determine the condition of all the components this can be a risky purchase because they are relatively expensive.
If these are a full excellent condition with no bushing wear, operable accessories they are worth at least $400, probably $500 IN FULL "E" CONDITION. Value falls with poorer condition, bushing wear or inoperable roar and horn.
Al, the prices Tom states are spot on as to the 490,492 diesels. I bought mine in 2014 at the Allentown First Frost train show. The seller was asking $500 for the pair but I bundled other items he had so my price was not the retail he was asking for the pair. These are in excellent condition and run like new and the horn and diesel roar both work. I already had a diesel horn generator so that wasn't part of my purchase. He also didn't have the boxes for them. Here are a couple of quick pics of mine on the shelf. If you notice , the 490 has the earlier 2 rivet steps, whereas the 492 has the single rivet one. Anyway, you can probably get an idea of what the condition is based on these. I wouldn't buy any diesel if they can't run it for you. The poorer the condition, usually means the more wear the motors and chassis bushings have. Hope this helps.