For a marshall half stack (a head plus cabinet), you're looking at about $1000 for the head and probably that for the cabinet as well. That's the price range for Marshall... it's obviously not cheap. Now, if you're talking a 100 watt combo amp as opposed to the half stack, you can safe a lot. I don't know much about the going price for a marshall combo like that.. or even if combos go up to 100 watts.
Can I ask what this amp is going to be used for? Gigging or just practice?,... because if you're just using it to practice in your room you DEFINATLY don't need 100 watts