Professional XLR input levels are +4dB, typically more consumer oriented 3.5mm unbalanced inputs expect a nominal input level of about -10dB, although this post suggests actually -15dB is more appropriate for the ATEM Mini for line level inputs:
viewtopic.php?f=4&t=111754&p=615921#p615921If you are just looking for a basic analog audio mixer, there are a lot of relatively affordable options. You might look at something like the Yamaha MG10XU or Mackie 802VLZ4 or I believe Soundcraft, Allen & Heath, Behringer and others make similar mixers in the $200-300 range.
If you want a digital mixer, these tend to be more expensive. I think Zoom, Behringer, and PreSonus have a few models with lower input counts in the $400-$500 range. If you are looking for something more portable, you could also look at the Zoom F4 or F6, or the Sound Devices MixPre line, although these might be a bit beyond your budget.