Due to the price of sabot ammo I rarely practice with my shotgun. I shoot before season to verify scope and thats about all. I shoot my other rifles and guns allot, but I feel your pain on buying sabot ammo. That being said, it's worth the price to me. I've got a mossberg 695 that shoots amazingly, and it's worth buying 3 boxes a year to me. One thing I do is buy the ammo on sale. Usually around january or so stores like Di**'s sporting goods will have good prices/ rebates on ammo. When I see it on sale I buy a couple boxes. Or you could spread out buying them. Instead of all at once buy one box a month throughout the year until you have what you need. I wouldn't shoot rifled slugs in a rifled barrel though. If you need to use the cheapy rifled slugs go back to a smoothbore.
-Jake