I have never looked into any of the cheaper options out there, but I get the sense many of them don't include a loadbox, just a cab SIM that requires you to have a cab/load plugged in at the same time.
Found a Palmer PGA-04 (ADIG-LB) for $350 CAD from a local studio guy almost 2 years ago, and it's been fantastic. It's totally analogue, has a cab SIM "filter" section and an unfiltered direct output, along with a loadbox built-in so I don't need a cab at all. Going straight into my interface with a touch of room verb, it gets pretty damn close to a real mic'd cab, quite a bit of tone shaping options, and I can juice my Mark IV as much as I want. I'm still toying with the idea of loading my head, the Palmer, and a power conditioner in an 8-space rack so I can do gigs without a cabinet.
Sounds like it might be just what you need. If you have a monitor in front of you, and the soundman can blast you out the PA at any volume, you're golden.
http://www.palmer-germany.com/mi/en/PGA-04-Speaker-Simulator-with-Loadbox-8-Ohms-PGA04.htm
No idea what these go for new, maybe $600+. But check it out.
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