Two comments:

1. The change from 53/39 to 50/34 gets you a 13% lower gear. The change from a 23 to a 27T rear cog gets you a 15% lower gear at much less cost.

2. If you need REALLY lower gears, consider going to a triple. They are typically 52/42/30 so a 30T chainring is a 23% lower gear than a 39 and it doesn't eliminate the smaller steps in the intermendiate gears. Even better, you can easily change the 30T granny for a 26T and make a great difference.