Just to put it out there, I was given the same advise as Louise. I would not advise what _rea78 suggested, not without the medication anyway.
If you aren't able to afford the medication, then to come off diet all together would be the best option. If you are on a low protein diet without medication, the effects would be far worse and she would be far more ill, then if she was to just eat meat, dairy, etc.
I'm still not sure where you are JW1990, but if you are in England, then your Mum CAN get a pre-payment certificate. This allows to spread the cost of prescriptions over the year. Its something like £12.00 a month and you get unlimited prescriptions for the year. However, if you are in America or somewhere, I know there is insurance involved and I wouldn't know how to advise that!
It is ALWAYS better to be on diet than off for a PKU person. But if the diet is just financially not possible, then it is best to be off diet all together, eating meat and everything is the best option.
I hope you found my website some help?! Again, add and message me if you have any Qs! x x x