I bought mine used on Ebay for $15 (looks like they are selling for around $30 now).
I should note that if you go to T-mobile's website, it says the free data for life plan only works with tablets. However, it used to work with tablets and some wifi hotspots including the one I bought. I figured I would take a shot at buying one of the old compatible wifi hotspots and see if I could get it to activate. Well, it did!
My backup plan was a karma router. (https://yourkarma.com/). It costs $150 upfront and then you pay for the data you actually use at a rate of $14/GB. The only downside to this is that the Karma router uses a captive portal to login and the Rachio doesn't work with a captive portal. I contacted technical support at Karma and they said they can add the MAC address of the Rachio directly to your account so you can avoid the captive portal.