The standard minimum campaign budget is $2,500 (or 2,500 in your market's local currency — for example £2,500 in the UK).
Depending on the publishers you'd like to pitch, a lower total budget may sometimes be possible — for instance where you're pitching a single publisher whose minimum sits below this. If you think a lower budget could suit your selection, reach out to your Linkby contact, who can look into whether it can be adjusted for you.
Each publisher also sets their own minimum, which your budget needs to reach before you can select that title — see How does the publisher's minimum budget apply? for more.