KnotScott can chime in on any current deals out there for good blades as he monitors ebay and others on the lesser known brands.
For a box store the Diablo line at HD will be the least expensive. I would recommend the 50T (I have that and it makes a very clean cut and is a good combo blade).
Irwin Marples for Lowes is pretty equivalent and a great blade as well, however I believe they are priced a tad higher than the Diablo. Check your stores to confirm.