Overtrousers are available for any budget. All waterproof versions are designed to keep you dry, but as a rule of thumb the more you pay the more breathable and more durable they will be.
Berghaus and Montane are perfect examples of this, although a bit more pricey, their range is comfortable and most importantly full of cutting edge technology and design to ensure you don't just stay dry but comfortable and supported too no matter what activity you are taking part in.
But if you are just going for a bit of a walk and after some more basic protection then you can't beat Regattas full range of waterproof overtrousers. From Under £10 they are the perfect short term solution at a lower cost.