Research by the Royal College of Psychiatrists in Scotland (RCPsych) found the level of spending per head of population for such services helping those aged 0-17 "varies significantly".
The biggest contrast was found between the Western Isles - where £156 per head was spent in 2019/20 - and Orkney, where only £20 was spent despite having a similar size of population.
While £44 was spent per head in Fife, a large geographical area, more than £105 was spent in Tayside during the same period.
And the figure for Glasgow and Greater Clyde was £90 compared with £103 for Lothian.
Dr Helen Smith, chairwoman of the CAMHS faculty at RCPsych in Scotland, said: "The level of spending on CAMHS across the country really is quite striking.
"Huge areas are vastly underfunded, while other areas with a lower population are receiving more money.
"It doesn't make sense that a child under the age of 17 gets a different level of service when it comes to their mental health, depending on where they live.
"We do welcome recent investment of £40 million, but it's only the first step towards ensuring there is equivalent resourcing for children and young people's physical and mental healthcare and as our figures show, disparities between health boards need to be tackled.
"That's why we're appealing to all political parties in the run-up to the election, to now look into how these vital services are funded across each health board - so every child and young person is given the best chance of treatment and support wherever they may live in Scotland."
Scottish Labour deputy leader and health and social care spokeswoman Jackie Baillie said: "We are facing a mental health epidemic in Scotland.
"It is incredibly worrying to hear that spending on CAMHS services varies so wildly across the country - the children and young people of Scotland must not be deprived of support due to a postcode lottery of provision.
"The last year has been difficult for us all, especially the young people of Scotland.
"Scottish Labour will produce an NHS recovery plan which places mental health services at the heart of our recovery and that will ensure no-one is deprived of the support they need."
Alex Cole-Hamilton, Scottish Liberal Democrat health spokesman, said: "It is heartbreaking to think how many vulnerable children are subjected to harrowing long waits for help. It is an utter scandal.
"Instead of supporting our hardworking NHS staff with the money and resources they need, this SNP Government has subjected too many young people to the risks of a postcode lottery.
"Scottish Liberal Democrats will put the recovery first. We are committed to a transformative investment in new services which can be quickly accessed locally, ending the waits of up to two years for help."
An SNP spokesman said: "During the past year there has been an immense strain on our mental health and on our mental health services.
"It will be vital for our recovery out of lockdown that we have robust mental services and the SNP has recognised that by committing a record £16 billion to the health budget, which includes an overall investment of £1.1 billion in mental health.
"Overall the SNP manifesto will set out further plans to improve the nation's mental health and support those who need it."