Yes I'd imagine different racks have quite a bit of difference on drag, the one I used was very large and blocky, thus it reduced MPG(I used it on a gas car) quite a bit. Speed would also make a big difference, going very fast(70+ as I did on my gas car) would cause a lot more drag than say going 40 or even 55 MPH.
I hear you, the little trailer I'm looking at is well over $2k and seem to be almost impossible to find used, hence why I don't have one yet
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a