Friends, relatives, charities and other benefactors are being deprived of up to 5% of their estate value which could be around £7,200 for the average estate, the firm claims.
A quarter of a million applications for probate were granted in 2010 according to the Probate Service.
Research commissioned by over-50s organisation Saga shows that 90% of older people choose a solicitor or financial adviser when it comes to making wills and dealing with probate, with some charging 5% of the value of estates.
"What's more, at a significant time of stress, when there's been a loss of a loved one, only 63% of the 11,000 respondents said they would shop around to compare services thoroughly, meaning they could well be losing out - cynics might say that some advisers are exploiting people's distracted decision-making at a time of difficulty," said Roger Ramsden, chief executive of Saga Services.
"The cost to families if they pay unnecessarily high charges could be astronomical - but it doesn't need to be."
Saga said it dealt with probate matters for a cost of as little as 1% of the value of the estate.
Ramsden said: “Some affairs can be complex, and tax and legal complications can lead to problems and stress during what is already a very difficult time.
“We would strongly advise people to get professional help rather than attempting to go it alone where probate is concerned but make sure you don't over-pay."
Around 500,000 people die each year.
Saga said even assuming only 10% have homes of average UK value - around £180,000 - and ignoring other assets, then commission or charges on probate matters involving the house alone could add up to £225 million for the legal and financial services sector even if they take just 2.5% and could be as much as £450 million if they charged the higher rate of 5%.
Ramsden added: "For the individual owning an average-value home, worth £180,000 it could mean the estate paying a commission of as much as £9,000 - easily enough to otherwise pay for the holiday of a lifetime, a new car, or a home deposit for a family member named in a will.
"That's a big number, and because wills and probate matters are literally once-in-a-lifetime for most people, few outside the legal or financial professions are expert - most people are unaware of the range of services on offer and how different the costs can be.
“Simple research can save hundreds, if not thousands of pounds, so we really do urge people to shop around."
