My kids have this and they love it - but I wish I had got it this cheap! The Galt Toys Marble Run is down to just £6.81 at Amazon, with free delivery for Prime members!
This set includes 24 different shapes and 6 marbles, plus a booklet with different designs of marble runs to build to get you started off. If the kids prefer they can just design and build their own runs, and send the marbles whizzing down the track.
This set seems to sell for around £13 usually, so at this price you're paying around half of that.
Delivery is free of charge for Amazon Prime customers, and on orders of £20 or more for non-Prime customers.