For others looking at prepaid data in Costa Rica, here's
Kolbi's page with prepaid internet packages.
I looked briefly at the competition (Claro and Movistar) and neither of them offer 4GB of data for 30 days at that price (9000 Costa Rican = $17 USD). Claro does offer 2GB for 30 days at twice the speed (4Mbps instead of 2Mbps) for 10,000 Colón, if speed is more important to you.
Given that Kolbi's subscriber base is much larger (83% based on
these numbers) I'm assuming they have the best coverage.