Hi yenemy,
I should also mention that you may need a foundation as well if you don’t already have one or if your bed doesn’t provide suitable support for the mattress. Inexpensive foundations that would probably be OK for a guest room are often available at local chain stores or mainstream retailers if you are buying a “mattress only”. There are also some inexpensive foundations listed in the foundation post here as well (especially the wire grid type combination foundation/frames).
Phoenix