The more you devolve, the more it costs to devolve.
Depending on the disease type, the initial cost can be negative (thus you get DNA back for devolving). This is why you got DNA back at first, and why it now costs DNA to devolve.
There are 2 genetic codes related to devolution costs:
This gene grants bonus DNA when devolving (+2 DNA Points for devolution). This gene is useful at lower difficulties because devolution of low-tier Symptoms, for example, frees up DNA Points that can be used to evolve higher-tier symptoms. In general, it reduces the total cost.
Costs to devolve don't increase. This is useful with Virus and/or high difficulty modes, because of high amounts of symptoms and high costs of symptoms respectively.
The initial cost of devolving also depends on the disease type. For instance, the bacteria refunds 2 DNA (you receive 2 DNA points) when devolving for the first time, while the virus costs 3 DNA points (you pay 3 DNA points).