I would imagine that the cheapest way would be to use a CEX. Could you recommend one to me? I know that OKX supports Linea but I have stopped using them since they implemented KYC. I need to transfer the funds from Polygon zkEVM but I'm flexible. I don't mind going through Bitcoin, or some other coin if needed as long as the process is as cheap as possible. I'm aware of bridges like Orbiter Finance, but that's a bit too expensive (for the small amount I'm trying to move).
If you have not exchanged yet then i will recommend Rhino bridge, They are providing zero fee bridge (maybe event or discount for limited time) . I checked out and Polygon zkevm, Zksync and Linea available for bridge. The minimum amount is $25. official website link belwo
https://rhino.fiIf you want to exchange lower than this then site mentioned by Libert19 is best one and I used many times for Starknet and Zksync bridge
Are you using these networks Zksync, linea for airdrop speculation purpose, If yes then I will recommend to use official bridge one because most of the time they require this step and also volume and number of transactions plays great role.