Every mail product that costs lots of money will yield a worse overall solution that using a good free/open-source mail software (postfix, qmail, exim... pick one) and spending money on people with good technical skills to tune and adapt the system. Unless, of course, your financial resources are unlimited...
It is not just financial resources - it is also a factor of time to build a filter / set of filters from scratch (even with spamassasin + bogofilter you need to train it extensively, and tweak its rulesets to suit your mail flow).
I think this part of question was referring only to MTAs, not MTA + anti-spam/virus tools. Anti-spam tuning is really a bit slower to do than general performance tuning (MTA or MTA + anti-virus), but this will be true to whatever MTA software and anti-spam one might buy.
Sometimes outsourcing corporate / isp mail handling to a provider like us, criticalpath, postini etc might be a good way to go.
Outsourcing is usually a good way to get a solution with expertise instead of a next->next->finish software installation and license to use it... but for ISP use, integration with internal OSS (billing, tech-support etc.) seems to be a challenge. Outsourcing costs also keeps most ISPs from using such a solution, unless time-to-market is the one and only criteria. Rubens