Visiting the Galapagos is not cheap, owing to travel restrictions and the remote nature of the archipelago.
Flights to the Galapagos are relatively easy to arrange and depart from Quito and Guayaquil on a daily basis for the Isla Baltra airport, about two hours by public transport from Puerto Ayora, the main settlement of the Galapagos, on the central island of Santa Cruz. There are also daily flights to San Cristóbal. The airport is a 20 minute walk from the center of Puerto Baquerizo Moreno.
Both Aerogal and Tame TAME have flights to the Galapagos. The price is the same for both companies, for foreigners around $350 from Quito and slightly cheaper from Guayaquil. Eucadorians pay half the price and there is a 15% discount on TAME flights and a 20% discount on Aerogal flights if you have an ISIC studentcard. Icaro also has flights to San Cristóbal
It's not possible to buy a one way ticket without proof of transportation from the islands. It's easy however to change the date of your return ticket or to switch your departure to the another island.
It is possible to travel to the islands by boat from Guayaquil, but in general this option is a major hassle that won't save money.