Yes, you can negotiate the price with the brand in the campaign chat and change your bid price. Before making a bid you should familiarize yourself with the brief, estimate the volume of the work to be done, and set a price that you feel will fairly reflect your effort.
If the collaboration already has a price set in the brief, it is not advisable to negotiate it since the price it's already there. If you disagree with it, we recommend you not apply to the collaboration.