Should I Charge My Customer More Because He Makes More?

Normally when I work with a client it tends to be a small service business like plumbers, roofers, electricians, etc..... and when I price them out for SEO I don't have a set price. This may seem like a negative to many individuals already; however, I price them based on the level of competition, search volume, and industry. The reason for this is that I see each industry and location as it's own separate puzzle and with that always comes new companies and problems you'll be put up against. In my mind it wouldn't be fair to say each industry is the same and each will take the same amount of resources to rank because that's just not realistic. Now here's the reason for my posting of this.

I recently posted about how I landed a client who makes a great revenue stream on his clothing brand. This client tends to be different than my normal prospects, but I'm always open to new challenges and after doing my research I thought I could help him accomplish his goal of increasing his SEO prowess across his website for $1500 a month. This was something that I saw as huge because normally my clients pay $750 a month or $500 if the industry isn't heavily competitive. When I posted about this many of the comments were something along the lines of "If hes making millions in revenue than you should be charging him more" now I see where they're coming from but also I don't feel the need to overcharge my client just because he makes more revenue. I mean you never know he could always be putting back millions into his business in product, paying employees, and a variety of other things. So is there something wrong with my pricing strategy? keep in mind I can always renegotiate the rate later down the line.