Is Yordle Explorer the card spent or printed cost when mixed with Sandswept Tomb?

Yes, Yordle Explorer checks the printed Power cost, not what you actually spend.

Multiple official FAQs (4773, 6912, 8453, 8812, 18) and Rule 131.4 all confirm: "Effects that need to determine a card's cost for any purpose always use its printed cost, even if that cost is altered or ignored as the card is played."

Sandswept Tomb gives a discount of [A] to spells targeting friendly units there, but Yordle Explorer doesn't care about that. It only looks at the Power symbols printed in the upper-left corner of the card. If the spell's printed Power cost is [A][A] or more, Yordle Explorer triggers regardless of the discount. If the printed Power cost is [A] or less, it won't trigger no matter how much you end up paying.