Like Albano said, if you think they (or the building, because that's where water goes when a 350gal leaks) could care less about money you will find out the hard way at some point. Whether the client cares about the insurance isn't really the issue anyway, it's the building who will require the certificate before you can do any work in there.
If an hour of maintenance is all you plan to do on a 350 and keep it looking good than go for it, at least they can't get mad about the money if it goes south. I normally bill around 175/hr but I'm fully insured and know what I'm doing, plus my group installs huge public aquarium sized installs, lab grade equipment rooms, and we can get just about any animal you can think of. If the equipment on that 350 is subpar you've got an uphill battle ahead of you, on top of the coming here to ask questions part.