it really depends on network. For example t-mobile will sometimes give out some licences for free (up to 5 i think) and you also get 1 free lience when you download blackberry professional server from RIM.
You basically have 3 products
BPS - Blackberry Professional Server (Cut down bes that can be loaded onto exchange server) up to 30 users
BES SBE - From my understanding this was what BPS was before it became BPS (someone please correct me if im wrong)
Full BES - All the bells and whistles but must be loaded on its own server.
It really comes down to what you will be doing with the product, if you just need basic bes functionality then BPS would be fine. But for the extra stuff mainly for large amounts of users you would want full bes.
Also im not sure which networks support what, t-mob did do BPS but are now saying its being discontinued with the release of 5.0 and in a Vodafone webinar i took part in rim/voda said they were getting out of the small business bes server, which sounded madness to me. But i would recommend checking the details with your network, but BPS should be fine, we run it here no problems. If you need any help with setup let me know as i have a fair amount of experiance with this now (and ive taken the server support certifications recently too :))