This is very strange, its like my partners ex wrote this... My partner moved away for his quality of life when he seperated from his ex. She obviously kept their child and he went every week to pick the child up/ see the child. That was his choice and he paid the cost for the travel as well as keeping up with all expected payments . He travels 3/4 hours on a return visit, so 6/8 hours in total for a weekend with fuel and still paying the CM.
However, until recently she decided she wanted to move, same reasons as you but further than they already lived. Making a one way journey 4 hours. So now my DH travels 16 hours , 8 hours return visit, with a 9 year old , still pays more than he should in CM and pays for all the fuel. She promised she would meet half way to help with the cost etc and she doesnt, shes done it once in 4 years.
Generally the outgoing of this cost a month is about £500 (with CM). There are other costs that go into the change of distance, such as car insurance increases due to mileage, risk etc as well as more regular servicing for the car doing almost 20,000 miles a year on this travel alone. We struggle financially , she expects nothing less than what she has been used to. Strangely enough she has recently changed jobs so now my DSC lives most the week at his aunties, He spends 1 night at home a week and see's his mum once a week. Shes not a reasonable person, expects everyone else to do everything for her and you just waste your energy and breathe trying to have any normal conversation.
My partner got into serious debt and had to claim bankruptcy as he couldnt afford on his previous wage the cost of this travel etc, however she completley lost it when he was to reduce the CM and for her to help with travel. So he had to put it all on credit cards etc whilst paying of one of her debts as well .... anyway.
He is in a better financial situation now, only just but we make do. We cant afford holidays etc but his DC is never without from our end.
If you are going to offer to travel etc then stick to your word as it has a massive knock on effect for the other parent and family. Only other option is to reduce seeing the child for longer stays in holiday times.