There were several M5s priced at around £16-17k when i looked around just after christmas.
I must confess i get a bit perplexed by the whole 'm' v 'non m' price debate..there are people quoting mpg figures and insurance quotes as the differential in running costs between the two, that's the tip of the iceburg and easy to budget for.
try looking at replacement brake discs, clutches and diffs...that's where the real disparity in cost lays, not 4mpg less in fuel economy!
If I went to look at a m car and it's had one owner from new and has been cherished, then, to me, it will command a premium - someone trying to use the lowest priced car on the market as a benchmark for another seller's pricing is a bit silly - go buy the cheap car off autotrader and put the difference in the bank, cos you'll need it!
same with parkers, i don't think a car would be on the market long if anyone used their prices..i've always looked on their absolute top book price as a decent guide for private buying!
I wouldn't touch an M5 for £16k, neither would i want to spend much less on a z4m, and, as an upfront cost, sometimes I do think the m is worth the extra over the 3.0, it's a never ending debate......