There are 2 payments. The first allows a lifetime license of PP allowing observation of ADS-B equiped aircraft. You can then sign up for a 21 day trial allowing observation of aircraft which have had successful MLAT requests made, this is basically a form of triangulation of aircraft that wouldn't normally be seen , i.e. Flybe and Eastern Airways flights for instance. Once the trial expires you can then pay around GBP10 per year to see these MLAT requests and I think you can also make your own MLAT requests, although not 100% sure about this. If you haven't already checked it out, have a read of the COAA PP home page :
PlanePlotter from COAA and sign up to the forum :
planeplotter : PlanePlotter
I do own an SBS1 and have never regretted the decision, it's an amazing piece of kit ;-)
At the end of the day, pay your money as you wont be dissapointed
Hope this helps
BAW1T