I have a Mossburg 500 which came with an 18.5 and full length barrels.  I am looking to do the same with an 870, so I could leave the 18.5 on around the house, but put the full length on for hunting.  Cost is a significant issue.  The cheapest 870 barrels I have found are made by mossburg, but still run $100 +, so I'd be looking at buying an 870 express for around $240 plus $100 for the short barrel.  Is there a cheaper way to do this?  Thanks.

Nope, unless you buy both/either one used. I've looked in to this in the past myself. Ended up getting two different shotguns in arfcom tradition.    

If you shop around, you can often find the Mossberg made 870 barrels for well under $100.

Also you can usually buy used barrels on Ebay and on the gun forums cheap.

Quoted: so I'd be looking at buying an 870 express for around $240 plus $100 for the short barrel.  Is there a cheaper way to do this?  Thanks.

You may have better luck doing that backwards. Buy an 870 with a short barrel and look for a used long barrel. They seem to be a little more prolific, consequently cheaper, on the used market from what I've seen.

Thanks for the suggestions, I will check E bay for barrels.