The absolute cheapest way is to add an
AIS fishing net locator from Aliexpress, about €100 with shipping from China. From what I understand, they do not receive AIS, only transmit, but you already have a receiver. I wouldn't really recommend doing this though, it's not really a good solution, just answering your question for cheapest.
The cheapest "proper" solution is either the Matsutec HA 102 or
em-trak transceiver (€250). You'd then replace either VHF radio with that, probably best the older one. Might need to clip a few mm off the top of the antenna to tune it for AIS (fitting a new one also works, but isn't cheapest!).