Found this on a German blog and wanted to share with you guys here:
DUS (or any German airport) - GUA with layover in MAD for €242 (€0.019 per mile = $0.029 per mile)
All flights are operated by Iberia.
Availability throughout March and April, but haven't found anything with less than 6 nights stay. I'm also having trouble recreating this fare on ITA, so I guess it's only bookable on the Iberia website.
To look for dates
click here
Edit: flights also available to San Jose, Panama City and San Salvador.