Hi, yes you want to make both applications together ... as is made clear by the application form AN(NEW). That is, there is space on the form for the child's details to be entered. I am assuming the dependent is a child? If not, ignore the above. But if not a child, what sort of dependent?
How much? That depends upon whether you are married to a British Citizen or not. Full details here :-
Details of fees
Timescales? On average about 7 months but some applications dealt with much much quicker than that. My wife applied in April this year and she heard just five weeks later that she had been successful. So application posted 15.04.04 ... citizenship ceremony attended 09.06.04 .. British passports held before end of June .. for wife who got Certificate of Naturalisation and also my step-daughter who got Certificate of Registration.
Finally, the language test requirement. That only applies to an applicant for naturalisation .... not one for registration. Accordingly there is no need to be concerned about a child's language ability.