RudderStack (https://rudderstack.com/) founder here. We have a different pricing vector (event-volume vs monthly-users/MTUs of Segment) so it's not directly comparable. Having said that, RudderStack can be much cheaper in two scenarios
1) Low # of events per users. For example, if you have a lot of anonymous users who don't do many events and leave. In this case, you may have a lot of MTUs but low number of events. True for a lot of consumer businesses.
2) You have a huge scale (10s to 100s of millions of events per day). In that scenario, we support deploying enterprise
RudderStack inside your VPC but still managed by us. In this case, we don't have to incur infra/data-transfer cost (and charge 80% margin) so can be much much cheaper. We have a couple of customers in this model. Some choose this for privacy/security reasons.
In normal scenarios too we can be 50% or more cheaper. Unlike Segment, we don't store any data (which is a major cost) so our COGS are quite lower. We use your warehouse for all the CDP features.
Which means your platform can't replay historical data to new sources, which offers a LOT of value for both onboarding new platforms as well as testing tools before signing a contract (since it lessens the cost of getting actual data into a tool to see if it's a good fit).