I really want to, but I got hundreds of things to do. / I'm tied up all day that day.
I am sorry but I have to turn you down.
I am sorry. I am not interested.
Thanks. Just looking. / OK, I'll think about it. / I can't afford it. / That's a little over my budget.
I do love it, but I'll need to talk it over with my wife/girlfriend.
There's too much workload here. I can't handle it.
There is nothing to talk about. / No comment.