If I MMS an iPhone user it costs me around 27p an MMS as my carrier (02) doesn't have an inclusive MMS thing in my contract. Is there any app that can avoid this - I've tested a few - SMSpro and Textra and it still get charged.
You can try to convince the people you're messaging to use a third party app like Whatsapp. It doesn't go over the SMS/MMS network and does not incur carrier charges beyond data.