How much do you charge for presentation redesign for a company? – Info Graphic Design

I’ve been doing some work for a commercial production company, basically renovating slide show presentations so they look good when they pitch to their clients. How much do you typically charge for this as I feel like I’ve been getting a little ripped off. Do you charge per slide plus extra for infographics? And how much do you typically charge? I know this is super vague but I just want some rough numbers to base my negotiation with them on.