No. I'd prefer to weigh up the financial implications of BOTH options, and then make an informed decision.
At the moment I'm PRESUMING that Costa would be asking for at least £100k a week (and him being a free transfer I'd not be surprised at all if he was asking substantially more) and something like a two-year deal as a minimum. I'm also PRESUMING that we could agree a deal for Dzeko that saw us pay a percentage of his existing wages, but would amount in cash terms to maybe £50k per week at most. And if we're paying £50k a week of his wages, we're probably not paying a loan fee as well.
Your use of the word "significant" suggests that you are PRESUMING Dzeko would cost a lot more than Costa per week... but you do have a bucket for a head, so I can see why you might think that.
Finally, if Costa does not perform for us then who is going to take him in the summer when we have signed him (in this admittedly hypothetical scenario) on a £150k per week deal? We'd end up paying his wages but loaning him out to some Turkish mob who would cover 5% of his weekly pay.