TheHUT is selling "Theme Park" for the Nintendo DS for £12.93 (inc Del.)
You can also get cashback from the usual sites (e.g. 5% Quidco, making it just £12.28)
Next best price seems to be £16.99 from dvd.co.uk.
Enjoy the challenge and excitement of the bestselling Theme Park series on the go for the first time, on Nintendo DS! Create, manage and develop your very own fun-filled park, packed with amazing rides, enticing shops and addictive sideshows, then add staff and entertainers all via an intuitive Touch Screen interface. Throw open your gates and let the public judge your efforts. Use your management skills to maintain an exciting, varied and safe park and your takings will soar broken rides, bad planning and a messy environment will keep visitors away. Balance the budget, keep shops stocked and develop new rides and features to ensure your park's success, then take your skills on a world tour, with unique attractions and staff to add a local flavour. With three difficulty levels and three game types to choose from, plus exclusive features and unlockable extras, Theme Park delivers all the fun of this unique simulation in the most accessible format yet.