Hi, Just5moreminutes
I'm the founder of Guess2Give so I thought I'd add to what's already been commented.
Womble is correct that our fees are higher due to the regulations that we have to abide by: gambling legislation, HMRC, age ID & UK residency fees as well as the card transaction fees. Despite all this bureaucracy and legislation the structure of each £3 guess is as follows:
1st guess, new user: £2.01 (inc Gift Aid) to charity, 50p prize fund, remainder in fees (listed above & our operating fee). This would only ever happen once as a new user.
1st guess, registered user: £2.28 charity, 50p prize fund, remainder in fees as above
2nd, 3rd 4th etc guess within the same transaction: £2.50 charity, 50p prize fund, remainder in fees
We have 117 charities on board including Cancer Research UK, Macmillan, Marie Curie right through to some sub £100k turnover charities. The feedback from the smaller charities is that they like us because we don't charge a fee which Just Giving does on a monthly basis, and Virgin charge as a setup fee. The charities never pay us a 1 penny.
Since launch we've had some great results which show that we're attracting new people to give. In some cases, people have a Just Giving page and near their event they setup a Guess2Give sweepstake as this attracts a wider group of people who would never give you a larger donation - such as your work colleagues, Facebook friends etc. Other people have said they didn't want to ask friends for money again via Just Giving and saw this a fun way of generating something for charity without having to have such a big 'ask' of their friends. Finally, people are setting up fun events such as how quickly can I eat 3 mince pies, guess the weight of the cake etc which can't be done on Just Giving, so this generates new money for charities.
We don't aim to compete with Just Giving. We're a game and completely different. Guess2Give is a bit of fun that brings in money from people who wouldn't normally support you ... and 67% of people who win doinate some or all of their winnings back to the charity which is also Gift Aid'able 
I hope that help clarify but I'm happy to answer any questions anyone has.
Cheers, Mark