Do you set a budget for each influencer?

Hey everyone! I'm going to launch my first web app in early July (lofi.co) and as the coding is almost done, i'm approaching a few YouTubers and Instagram influencers to schedule promotions in the days following launch. Out of the 15% of them who respond, the vast majority is able to give me a flat rate or even better a media kit, but there's also a few who ask 'What's your budget?' How do you guys approach them? I have a set marketing budget but i don't like to have one for each influencer i contact. Who knows your audience better than you? And also, these days influencers with the same amount of views, in the same niche and with the same audience location can charge one 50 times the other for no apparent reason other than not actually knowing what they're doing. You can actually find someone charging $5000 for a 40 seconds promotion in a video and someone else with the same analytics do it for $300-$400 (this has actually happened, the only difference was that the first one was managed by an agency). Let me know what you think!