Is it considered unethical to pay participants differently based on some quality? Let's say you are recruiting two groups of people, English monolinguals and bilinguals. If you're having a hard time recruiting enough bilinguals, the natural, free-market thing to do would be to offer to increase how much they are compensated to attract more.

Assuming that both groups complete similar activities in the same time frame, is it unethical to pay one group more than another?

Would the answer be the same if the quality wasn't the part of the design? For example, if your monolingual group is older on average, is it unethical to post an advertisement for an increased rate for young monolinguals to bring your group averages back to comparable? 

I have never seen a research group do this, presumably because of how time-consuming it would be to do, but I haven't found anything to suggest that it is wrong to do.

I'm asking here because I briefly discussed this idea with someone, and she seemed to think the IRB would take an issue with it but we didn't got much further.
I don’t see on what grounds anyone could regard this as unethical, but one might legitimately wonder whether paying the two groups of participants unequally might inadvertently create a bias that would compromise the integrity of your conclusions. After all, your research is presumably premised on the assumption that the participants from the two groups are both fairly typical representatives of those groups. But if you pay participants from one group $10 and those from the other (for argument’s sake) $5000, the participants you’re likely to get in both groups might differ in some ways that you didn’t expect - for example, the lower-paid participants might be people who are more motivated by an altruistic desire to help scientific research than the higher-paid participants

Whether this is a genuine concern or not depends on the difference in the pay and on the precise nature of your study, but it’s something to think about.