im a guitarist of 4 years now and im thinking of taking up the bass as well. but im short on cash right now so i was wondering how much i should spend on the amp since i only have a marshall 10 watt. also, will the bass amp be good for playing my guitar? because i know that bass signals are stronger so the amp should be able to handle it, and if i could sell my amp the money would help me get a bass.
You don't want to use one amp for guitar and bass (unless it's an Orange, but I'm going to go ahead and say that you can't afford one of those). I'd recommend getting a bass starter pack, like the Ibanez Jumpstart pack. Then save up for a new guitar amp, and always keep guitar and bass amps separate.
Amps with a preamp circuit VERY closely resembling the original Bassman circuit work very well for both, however they don't push a lot of wattage (as far as bass amps are concerned) and are generally rare and/or expensive. These include the Bassman, Marshall JTM's, The Traynor YBA's, Orange Rocker's, and few others.