If you find the right nursery, buy them in the fall (best selection and planting time then, I think), and/or get them on sale (also common in the fall), you can often get what plants you're looking for for half that price! I'd do a google search once you know the specifics you're looking for, and if you want to check out the vendor recommendations on my profile, most of them are where I've found good plants inexpensively online.
Personally, I'd go for a shade-happy shrub as the main event, rather than perennials. Or intermixed with perennials. I like having the permanent fixture there all year long. You might do a google search for the qualities you're looking for (ie: 4' flowering shrub for shade zone 7), and see what sounds best!
