Could any of you recommend a good hostel in Antigua? Less than US$20...

A few options here.

The above list is a good starting point, but if you are not going to be here from Christmas to New Years or the Easter Week, I would suggest that you just come on to Antigua and look around for yourself. You will easily find a place to stay without a reservation. Most places that charge less than $20 don't make reservations anyway. There is such a variation in the different hostels that you should check them out yourself. Also, talk with other people you meet for recommendations as to the type place you will want to stay (party atmosphere, quiet location, etc.).

You will find a place you will enjoy staying. Antigua is the traveler's hub of Guatemala.

Posada Juma Ocag is excellent, and one of the best values I've ever had for accommodations. I think it was 140 quetzales for a double. You can find much cheaper places to sleep in Antigua, but nothing this cozy for so little.

Felix is right about the value you get for your money with Juma Ocag. The place has a great location, nice and quiet, very clean, and safe. It is where we would stay every year while we looked for a place to rent long term.

Get there early in the day for a room, as it fills up quickly. Also, ask to see several rooms, because some of the rooms are larger than others.