I don't agree with the logic behind charging different customers different prices based on their revenue. If I want to buy a car, the price is not determined by my income. Same with a house. Same with legal services. I guess in some extremes that's not the case (subsidized housing, state-provided legal resources, etc) but in general the price is the price and your income has nothing to do with that. Charging customers with less revenue a lower rate is, like it or not, a subsidy.
If you did provide lower rates to customers with less revenue, your higher revenue customers would be quite right to ask you why you're charging them more, if they ever found out you were doing that. Pricing practices that I can't defend if they become public are, in my mind, not a good idea.
That being said, whatever rate you pick, here's one bit of advice I'd very strongly encourage you to consider. Pick your normal rate, the one that applies with a (say) 3 business day turn around. Multiply that by 1.5. That's the priority rate - within 24 hours. Multiple the priority rate by 3 That's the emergency and weekend rate.
So, with $50 as the normal rate, the priority rate would be $75. The emergency/weekend rate would be $225.
The idea is, a single unreasonable client who wants everything "right now!!!!" can ruin your customer service for everyone and make your life miserable. Make "Right now!!!!" extremely expensive relative to your other rates, and incentivize reasonable expectations on behalf of your clients. You'll sleep a lot better at night if you do that, it'll be easier to stay focused on school (since you're not constantly getting "right now" requests), and (if your experience is anything like mine) your clients will understand and appreciate the rate sheet. It does in fact protect reasonable clients from very unreasonable ones, and it gives clients the option to prioritize things when they really, really need to.