The word they’re looking for in this advert is whatever not what and ever separately.
In this context, the word should be a pronoun – meaning ‘no matter what’ and is one word. The form that comes out here is interrogative so: ‘What ever (is) your budget?’ which clearly doesn’t make much sense without an additional verb.
A rule of thumb is if you can remove the word ‘ever’ without ruining the meaning then using two words is fine. In reality, use of ‘what ever’ often comes across as a little bit affected and formal:
‘What ever is his problem?’
‘What ever did you see?’
and then there’s the rest of this advert… *sighs*