How much would a train-ticket cost from Taunton to Brighton?

You could try here: http://www.qjump.co.uk/

Depends on what you want. I'm assuming that you want an off-peak (i.e. non-rush hour) return (i.e. there and back) in second class. I'm also assuming you have no mitigating items such as vouchers, deals or railcards and that you are classified by National Rail as an adult. In the case above, the lowest cost (N.B. this may involve changing trains more than a more expensive journey and/or a longer journey than a more expensive one, so decide for yourself what you'd like) is £42. The source takes you straight to the results. If you wish to change details, it's eminently feasible to do so on the website.

Go here http://www.nationalrail.co.uk/ and fill in the details

Hope this helps: http://ojp2.nationalrail.co.uk/en/pj/fa?uniqueId=25702328

go on railtrack.