Hey look I love my RZ, but there are things it won't do. Split-second snapshots and night portraits are two examples, hence I still have a DSLR with 85/1.4, AF and stabilisation that I can shoot by the light of a single candle. Sure it's not going to print out nearly as good as 6x7 Acros or something but the latter just does not have the sensitivity to get those shots. Use the right tool for the job - sometimes it's medium format, sometimes it's large format ... sometimes it's not.
But yes, you can do portraits with the Mamiya (see all the good wedding shots taken 1960-2000) but it takes practise, forethought, preparation, and more practise. Flashes and good light are a huge help because they mean you can stop down a bit - a small (60cm) softbox with a good hotshoe flash in it can achieve f/16 to f/22 at 1m, which means an easy f/16 at 2m and ISO400 (flash as main light). If you're in bright open shade, that's about f/5.6-light, so you shoot at f/16 1/125 and the shade light becomes your fill at -1: instant beautiful outdoor portrait, just add 81B! You can set it up and work that one exposure with no further thought for a couple hours, with enough DOF to cover minor focus issues.
And $1.50/shot? That's like $23/roll on 645 - what are you doing!? Even in Australia (the land of incredibly high prices), I can buy C41 from the US for $5/roll and pay a lab to develop it for $7 (DIY for $2). Cheaper still for B&W.