Polar s720i with speed, cadence, and IR interface $333
The Polar s725 that you mention adds minimal functionality over the s720 (can link to some specific Nokia phone)
Polar s725 $297 (includes speed sensor)
If you are planning to purchase power option (which I think works well having had if since X-mas- but many others will disagree) -think hard about getting the s625x- the power sensor includes speed and cadence sensors- there is no point paying for the s720 or s725 (which comes with a speed sensor) only to not use it. You can have the foot pod speed and distance monitor for the same price!
Polar s625x (includes speed and distance foot pod): $315
Polar s625x with IR: $350
Polar Power Kit: $315
In my opinion- s625x plus power plus IR = $665 not a bad deal (and my experience is that you can negotiate an even lower price with the retailer).
Hope this helps.
By the way- my experience is that using the internet to look up prices is at least as easy as placing a post on slowtwitch