uneven your question is a tough one to answer because there is nothing that is normal. Commissions are not set by law. You can go to alot of online places and look at the commission structures that they have and maybe they work for you and maybe they do not.
There is no normal. What a realtor sees from a commission varies alot and is 100% determined by where he works. Many large brokerages take a good percentage of the commission so the leftovers the realtor gets makes it hard for them to give money to the client.
Smaller brokerages and independents, may not take a large percentage leaving the realtor more leeway to give a rebate. Many of the online places don’t use realtors but just have people let you in the door to a home and pretty much you are on your own but they may give you a large rebate.
A somewhat reliable rule of thumb is that the rebate will vary with the work you expect to receive from the agent. Do you want comps? Do you want advice from the agent? Do you care what other homes in the same neighborhood sell for? Do you want to know if the agent sold or represented buyers in that neighborhood? Do you want help with inspectors and/or contractors? Do you want research or foreclosure information? What do you want from the agent? Some people want more and some want less. You can kind of set the parameters and talk about them with your agent and then come up with a sensible rebate amount.
There are alot of options out there for you with regards to rebates. If your previous agent gave you 2500 on a 450k deal then that was nice of her, specially without you asking for it.