Are there bolt receiver holes on the spider you have now, that would take a 3rd chainring? My bike came with 2 chainrings (like yours). I needed a granny gear, and my shop was able to just bolt a 3rd ring onto my spider. I think the one I bought cost $19 or $20, and the guys didn't charge me labor to add it on.

(might be cheaper than buying an entire crankset, if your spider has the space for it)