The exam entries themselves are usually not too much. Tuition is very variable depending how much is being offered and of what quality.
The most expensive part of taking A-level science subjects as an independent student is the science endorsement (practicals). These are normally required by most unis if you have taken A-level sciences, and can easily cost over £1000 per science. They also aren't always widely offered, so you may need to hunt around for a provider for them.
Depending what your aims are, a degree with a foundation year, or an Access to HE course, may be a better and cheaper option. However A-levels may be the better option in some circumstances still (e.g. a foundation year isn't available at the uni you want to go to, an Access to HE course isn't accepted for the course you want to apply to, etc).