Connect to the Tower Records website. Connect to the Dress Circle website. Decide for yourself if they are worth the cash, though to be fair, the quality is usually not bad - often pretty good. All the stuff looks like it is made by one factory, only the show logo varies. They can also mail goods if you forget to buy on the day, but write first rather than telephone.
Cast recordings are usually cheaper from high street or internet stores.
The mail order price in programme advertisements are usually high, especially when postage is taken into account, and the foyer price can also be outrageous. Very few soundtracks and souvenirs are collectable. Only recordings and items from failed shows or surviving from the earliest days of a long running hit accrue value. They usually must have been circulated in limited quantities, and be in mint condition. Theatremonkey puts the free flyers advertising shows before they open as a better investment than most merchandise.