The Department of Trade and Industry has announced post offices in the region will receive £60m over the next three years as part of a £450m national package.
But local post office managers say businesses will continue to go under - and the cash injection is "an insult".
Mark Baker, executive officer for the National Federation of Sub-Postmasters in the South West, said: "We have already seen a loss in income as a result of a drop in revenue from paying people's pensions.
"If you include the overall loss of income, post offices will still be shut down.
"Many postmasters in rural areas are paid less than the minimum wage - and if they are kept at this level for the next three years they will not be able to continue."
The government says it is showing real commitment to rural post offices.
But Robert Gibson, who has run Budock Water post office near Falmouth for 11 years, said his salary of less than £15,000 a year would continue to fall.
'Social place'
He said: "There's a feeling among postmasters that the money will be used to subsidise the very small offices, just pushing up the sub postmasters' salary to cover the national minimum wage.
"Post offices like ours may see nothing and in fact may see a fall in salary because we will be losing the benefits business in April and that is about a third to half of our business.
"We are seeing huge rises in house prices and I think postmasters will suddenly say, 'I can turn the shop into a house and I can sell it for a lot more than I could ever get for it as a going concern'."
Many rural post offices offer much more than the usual services.
The St Germans office in Cornwall even offers dry cleaning and laundry.
Office manager Sue Prowse said: "It's a social place, people will come out once a week for their pensions and maybe they will not go out at any other time, so it's the only time they meet people."