I can't see why you'd need IEM for a solo harp and voice.
If you need monitors at all (which is debatable), a simple speaker turned up to a level where you can hear it should be enough. If it's fed with the FOH mix, you can use it to check and control balance between voice and harp.

Monitoring in general is to solve the problem of not being able to hear yourself, or other players in your band, because of high levels of noise on stage. The presence of a drum kit or instruments with on stage amplifiers is usually a good indicator that monitoring will be needed. The additonal advantage of IEM is that you can have it as loud as you like without feedback problems.

None of this is applicable in your case, is it?
Also as Backwoodsman mentions, it's expensive...