The Family and Childcare Trust (FCT) said the increase in Wales between 2014 and 2015 is far larger than rises in England and Scotland.
It also warned that there is not enough holiday childcare to meet demand.
Welsh councils said they are "working hard to support families in need of childcare, including during the summer holidays".
The FCT surveyed local authorities in Wales and found the average price of public sector holiday childcare, such as schemes run by councils or in schools, had risen by 69.9%.
The average increase between 2014 and 2015 across Wales, England and Scotland was 14%.
Jill Rutter, who carried out the survey, said: "Welsh local authorities have until very recently protected a lot of their public sector childcare.
"In England the cuts started much earlier, back in 2010, now things are really catching up on Welsh local authorities."
The average price of public sector holiday childcare in Wales is £108.31 a week. The average price in the private sector is £117.13 a week.
The Welsh Local Government Association (WLGA) said it is working with the Welsh government and childcare providers from the voluntary and private sectors to "consider the issues that parents face in finding accessible and affordable childcare".
The FCT said 95% of Welsh councils lack sufficient holiday childcare for working parents and not one provides sufficient childcare for disabled children, compared to 87% and 88% of English councils respectively.
A spokesman for the WLGA said: "Local authorities in Wales carry out childcare sufficiency assessments which highlight any gaps in childcare provision.
"There is, however, no legal obligation for councils to provide their own holiday childcare."
The cost of council-run school holiday clubs has increased by nearly 70% this year, according to a charity.