Pass the crystal ball please..........
About £700 per person should provide an inside or maybe outside cabin on a 3 or 4 star ship. £850 may secure you a balcony but you dont give much detail in your question so you cant expect a detailed answer.
Sounds ok for a 7 day cruise if last minute (like the next day) for an inside cabin but for an outside, hmmmmm. Again depends on the cruise line, where you are going to and what you are getting.
I assume you are in Laredo, Texas which makes me wonder why you are enquiring on a UK cruise web site about cruises in the Caribbean. Shouldn't you be looking on the .com website rather than the .co.uk one?