From the Zona 1 bus station in Guatemala City, here are the latest prices for Linea Dorada when one buys direct from the source. Luxury and first class is theoretically a different bus, however when I took it they used the same double-decker for both classes with the sole difference being priority boarding and a dinner consisting of a hot dog and soda. Although the bigger seats and recline of the luxury bus for an overnight trip is probably worth the extra $5 USD, check the seat maps for both buses at the ticket counter if you have a first-class ticket before choosing to upgrade or you may find yourself paying extra for the same services. The bus has no security, but for a reason I am not sure of people consider Linea Dorada the safest in El Petén.
In contrast, the cheapest ticket in Antigua I could find for first-class Linea Dorada to Santa Elena/Flores with a shuttle service from Antigua to Guatemala City was $30 USD / 240Q. With a difference of 90Q, I suppose you are paying the premium for the shuttle service direct to the station. Given the area of the bus station (the graffiti, lack of people, and history of crime), it’s probably worth the premium.
For Santa Elena/Flores, the bus leaves at 9AM and arrives at 6AM, making stops at both cities.
Edit: Having trouble getting the piture to load; its 150Q for first class and 190Q for luxury from Guatemala City to Santa Elena/Flores.