Microsoft Store (online/retail locations) were running this promo a few weeks back. While there was much discussion about the low trade-in values for some devices, if you have an old Surface RT laying around (like I do) they'll give you $150 for it towards a SP4. You don't need anything more than the AC adapter. If you have the type cover you're better off selling it on EBay or giving it to a friend as including it in your trade-in does not change the value.
Surface Trade-In ? Microsoft Store
Surface Trade-In ? Microsoft Store
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