Hana, You are about 25 min drive from La Fortuna, but yes, you should be able to do better than US300 one way from Liberia airport.
You transport options might depend on how many ppl you have. For 2 ppl, you might be able to arrange a ride in a shared shuttle van (e.g., https://www.interbusonline.com/rates. There are others, too). They operate on schedule.
If you have 3-4+ adults, look into private transfers. Those are pd per ride, whether you are 2,3, or 4 ppl. If you have a larger group, 5+ ppl with luggage, travelling together, you can look into vans of different sizes from https://www.morphovans.com/liberia-airport-shuttles/order/ .
Re “Is flying a better option?”: The domestic air company Sansa doesn’t have direct flights between Liberia and La Fortuna.
If you were flying into SJO, there is currently only 1 ticket left on the 12 pm flight leaving SJO to La Fortuna, via Tortuguero (total flight time is an hour and 10 min.), on Mar. 1, 2023, for US101 pp one way (https://flysansa.com/). So not ideal for you. Not to mention that you would still need to add a taxi from the local airport at the destination to the actual hotel.
Are you planning on having all your meals and doing the tours offered by the hotel? The reason I am asking: since you are looking for less expensive transfers, you might want to read this review: https://www.tripadvisor.ca/ShowUserReviews-g1453880-d643437-r873810133-Chachagua_Rainforest_Hotel_Hot_Springs-Chachagua_Arenal_Volcano_National_Park_Pr.html .