I see two possible solutions:

The cheaper route would be to just get some spd pedals and another set of shoes for your "old" bike.

I have Forte (Performance) spd/rattrap pedals on my single-speed and a pair of sandals with spd clips. When I want to go out on the singlespeed to run to the store, I just slip on the sandals and go. Different bike, different use. I also have an inexpensive pair of mountain bike shoes with spd cleats for when it's too cold to wear sandals.

The other route would be to sell the excellent but expensive Speedplays on ebay or craigslist and buy 2 pair of something cheaper, like Look or Ultegra. The $100 Ultegras at Performance are a steal right now, plus there is a 10% off coupon going and free shipping over $75. I got mine using a 20% off coupon, so they were only $80/pair. I was also able to sell my 2 pair of old Look pedals on ebay for $66 and $80, so my upgrade only cost me $14!

Just some ideas for you.