The simplest way to keep the cost of beautiful flower beds down is to take advantage of nature's habits. Perennials cost more in the short term but they come back year after year making the cost over time negligible. Speaking of time, perennials are a time saver as well. Once the first growing season is past, they don't require nearly the same amount of time and attention that annuals do.
Bulbs are another great choice. Some need to be taken out of the soil after they bloom so be sure to read the packages. I avoid these. There is just no time in my world for high maintenance plants! Daylillies are beautiful and they not only come back year after year, but they spread as well. This allows you to lift them out, seperate them and move them around to cover a larger area, if you choose. Last year a friend decided to thin her daylillies and I was given the spares. They looked a little anemic but I planted and mulched them, hoping for the best. They have come back this year with gusto. I've had good luck with Iris as well.
Flowering shrubs like azaleas are also a favorite of mine. They are hard to kill and easy to please. The only important thing about azaleas is they set their buds in the fall for the following spring so beware of the urge to prune in the fall. Late pruning will result in no flowers in the spring. I learned that lesson the hard way.
I don't really use annuals, they aren't usually cost effective in my opinion. The exception to this is the impatien that I got for Mother's Day three years ago. Cuttings from that original plant have been used to fill window boxes, hanging baskets, and regular pots. That is the great thing about impatiens, they are so easy to root.