Do you need the calling minutes more, or the data more?
If you want lots of data, Wal-Mart has a T-Mobile card @ $30/month. I believe it comes with 100 minutes/month and 5GB of data.
If you need more minutes, the basic Project-Fi plan is $20 for unlimited minutes, $10 per 1 GB of data (pro-rated - you pay for what you use).
There are reddit threads out there that detail which "unsupported" phones work with Project-Fi. That said, the cheapest supported phone is going for about $199 right now (Nexus 5).