Bulk raw wafers can be obtained for a couple cents in 120k quantities but the tooling to apply them is where the cost is mostly. Also Walmart is a huge customer but not the sole customer for the brands they stock so they'd have to convince a lot of different companies to include it to make it worth while.
I think the second is a big part of why RFID tags haven't taken hold, stores need enough of their stock to use them to be useful while manufacturers need enough clients to want the tags embedded to go through the trouble of including it in the packaging.
This is a genuine question by the way, I honestly have no idea of the scale of this when applied to a store chain as huge as Walmart.