@ryder I have two Rythmik F12 Direct Servo subwoofers (Signature Edition) which I think are much better value than REL subs. They're $1260 each and are amazing. I got a REL 328-R used for about $900. I use all three. The ratio of these subs together is about 1:1 with my speakers. I could probably do without the REL, but the multi-sub approach works well in my room. At some point, I will get better speakers and the ratio will change, but the subs are here to stay.
In other words, the ratio of sub cost to speaker cost is hard to judge unless one controls for value (value is the ratio of sound quality to cost). In my estimating, having three REL subs would be way too expensive, ratio-wise, for the speakers I have.