If you keep your eyes open for a good low cost temp controlled soldering iron, some needle nose, side cutters, and a cheap Digital Volt Meter, you can probably get kitted up for under $60. I would also suggest a Solder puller (desoldering device) for the inevitable mistakes/repairs that do come up. That would cover you to do the basic circuit building.
Casing is another story, but as long as you have a decent handheld drill, you can do most of the smaller holes for connectors, volume controls, etc.
Once you get into doing larger holes, IEC connector outlets etc, you start needing things like stepped bits($50), Dremel type tools(for squaring off holes, etc), which gets pricier.
I snagged a nearly new tabletop drill press for my case work for $60 a while back, its been very worthwhile for me(working on my 4th amp).