Passing this along for users of the $30 (or any) T-Mobile plan, and I didn't want to start a new thread.
Target periodically has TMO prepaid cards on sale; I've seen it twice in the past 3 months. (I don't usually walk through that aisle, but I am making a point of it now). It wasn't a lot off, $2 maybe, but hey every little bit counts, plus I'm still leery of buying cards on eBay. I have a Target "red card" and using that, I saved another 5%.
I couldn't find a sale this month, but here is something else interesting that I discovered - when I buy through Target, they don't (so far) charge the 911 tax that my state recently instituted. And more interesting, I just bought a card online and they didn't charge sales tax. I would have thought they would, since they have stores here, but I saw someone mention this on one of the online reviews and I tried it. (I saw some online comments elsewhere that indicate this varies from state to state). So even without the sale, I saved $3.90 on the $30 card. FWIW, on paper I also saved shipping costs (4.98) with the red card, but don't count that because I wouldn't have bought it if I had to pay shipping, although for a bigger denomination card it might be worth it. (ETA-just being clear, I still ended up paying $28.49, but $3.90 off of what I would have paid at TMO online).