Ex MAD, there have been cheap o/w as well as r/t flights in basic eco for some time (OW and *A). E.g., MAD-MIA non-stop on UX is EUR 151 o/w when booked directly through UX (good availability outside summer vacation period). MAD-MIA nonstop op. by AA is EUR 136 o/w when booked directly through AA (April through June 2021 only).
There are also r/t flights such as MAD-LIS-MIA // MIA-LIS-MAD on TP metal for EUR 214 directly through TAP. Good availability outside the main summer vacation period (= late June through the end of August).
Ex AMS, we often see cheap basic eco fares as well. They are currently back. It's mostly on OW.
AMS-PHL-MIA from EUR 186 o/w or EUR 286 r/t. Note o/w are only available until June 20 or so. r/t available until EOS except for the summer. Search via GF, book through AA or BA.
AMS-DFW/PHL/LHR-SFO is cheap as well, r/t from EUR 292 outside the summer.