As mentioned many times over, years of collecting helps take the sting out of the total cost of things. We've always had Disney ornaments, but about 10 years ago we realized that with all the spending we'd been doing at the Christmas shops in Disney, we had more than enough to do a Disney only tree. At this point, we could easily do two whole trees with nothing but Disney.
The individual ormanents are the most costly, obviously. Not cheap, but a cheaper way to fill a tree is to look for the packages of similar/same ornaments. So the Mickey head versions of the traditional glass ball, we've picked up the drawstring bags full of balls with different scenes on them. . things like that. The cost per ornament would break down to less than buying them individually so it helps you stretch a bit until your collection of individual ornaments builds up.
Your best bet would be to shop the after Christmas sales and build your collection for next year. Most stores that carry decorations will carry Disney ornaments, so if you can hit the clearance sales and stock up you'll be able to stretch your dollar even further. The
Disney Store usually start marking things down fairly early - we usually end up picking a few new ones up at half off before Christmas. I rarely order online, but I've noticed some really fantastic deals in the outlet section around the holidays and at random points through the year.
The closer you wait til Christmas, the more sales you'll find. When everyone's shopping mode changes to desperate search for gifts and their houses are as decorates as they're gonna get so the decoration aisle is no longer on their radar - you'll see the prices start falling.
A couple have mentioned Hallmark. I used to work at one in college and when they had their middle of the year clearance at 75% off, I got first dibs (since I was the one putting them out, LOL) and picked up dozens of Disney ornaments for next to nothing. So watch for the sale and try and hover around the store when things are going to be put out because the best ones go quick. I had a few people standing over me as I was putting things out - some of the styles were "sold out" before they even made it to the shelf.
That $10 store is also good for having a few different styles on clearance. We too picked up some that were 4 for $10. I saw someone post, asking where it is. . . it's in Downtown Disney, next to Pooh Corner near Goofy's Candy Company and across (and a little over) from the Christmas Store.