"Unfortunatley they all tend to be somewhat expensive."

Yes, I've always thought that; especially when compared to the price of USB-to-RS232 Converters - which seem to me to be doing a broadly similar job!

[Cynic]
As I mentioned before, they originated in the land of mainframe/mini computers - so, presumably, manufacturers are just setting prices according to the expectations of legacy mainframe/mini computer users; viz, expensive!
[/Cynic]