That's pretty much it. The more you use the more you pay. Right now, Morty is only offered as a prepaid plan. You add money to the account up front, and the cost of the services are deducted each month as you use it. The first 10k CPU and 100GB RAM each day are included in the base rate of $1/month, so if you never go above 10k CPU or 100GB RAM on any day during a month, your account will only cost $1 for that month. The overage rates above apply for usage in excess of those limits.
If the account runs out of money, it moves itself back to Tommy (and if it's overusing, gets suspended for high load).
Load is calculated the same way it is in Tommy and Johnny, sampled every 60 seconds and summed up over a 24 hour period. You can view your load here: https://heliohost.org/dashboard/load/