That looks like a nice little 486 system. Prices on eBay (for just CPU units) vary quite a bit. At a glance, similar items can go for perhaps $40-$130 dollars, but that depends on a lot of factors.
A few thoughts:
Pop the top off to determine if there are any special cards installed (I doubt there are but it is always worth checking)
Is that an AT form case? Usually DELLs have propitiatory form factors, but if can take a generic AT motherboard, that would be of more interest to people.
Is there a barrel battery on the motherboard? If so, has it leaked, damaging anything? Or is there a Dallas/Odin integrated clock battery, and if it does is it soldered on to the board? If it has a simple coin cell battery instead, those are the best.
That 5.25" floppy drive is a plus. If you do decided to trash the system, at least remove the floppy drive. That would be an easy ~$20-$30 on eBay. One can never have to many floppy drives.
The keyboard is just a standard cheap PS/2 Windows keyboard, if it works with the CPU unit, just include it with that.
The monitor looks decent, but nothing special. According to the specs it should be capable of at least 1024*768*85mhz. But the more desirable VGA CRTs are usually late 90s, early 00s, or special early models like from IBM. Shipping is usually prohibitive. If you offer the set for local pickup via craigslist or similar you might get some interest.
You can forget about that back-ups. The battery is probably dead or near dead, and just a huge paperweight. You can still buy similar devices.