Conversion rates for membership

Hi there - we are a UK based conservation charity planning our first major PPC campaign for online membership recruitment.

Looking at the Google results for our proposed keywords we can see that many do not have enough usage data to give us comprehensive bid statistics, but for the words we can get data for, the average seems to be around £0.30 to £0.50 cpc and still a very broad estimate.

The other variable factor in calculating our overall potential ROI, is the conversion rate of the online shop. Having never run this type of campaign for membership before, we can again only estimate based on current conversion figures.

Our shop activities have 3 distinct product areas with conversion rates ranging from 0.7% to 2.9% to 4.9% - the higher figure reflecting the high use of the shop for fulfillment from offline marketing and customers entering the shop having already made the commitment to purchase.

So I was wondering if any one with membership organisation experience would like to share PPC costs and conversion rates to help us gain a better idea of what to expect.

Many thanks