It very much depends on the type of society - academic societies that just organise a few socials will usually only be £5-10 for the year. However, sports societies that have their own equipment (and sometimes premises) to maintain will often cost in the region of £70. However, I'd argue that the sports society will give you a lot more for your money (use of equipment, coaching, socials, trips abroad, away matches) for your money than an academic society ever will.
If you're nervous about spending a lot of money on society membership, then find out if they have taster sessions and / or temporary membership so you can try out the society for free / cheaply.