I used to have a mixer similar to
this one that I found on Ebay. I would use my laptop to feed the mixer then use the output of the mixer to feed an amp for a DJ setup. The mixer has a headphone jack on the front that you can use to monitor the output of the board or whatever channel you switch to "cue" position. The headphones can be turned up pretty loud and are after the equalizer, so any equalization you apply is heard in the headphones.
The one I use now connects via USB instead of the audio jack, but is way more that the budget you mentioned. I have no connection to this Ebay seller and have no idea how trustworthy they are, but this serves as a good example of something you can look for used that will do what you need without the need to buy additional equipment other than possibly a cable to go between the laptop and mixer.
If a software equalizer that runs on your laptop will not do what you need, this may be something that could work. You might be able to use it for other thing besides your laptop as the source.
