Health commissioners said they expected the capital costs to remain at £312m.
However, Mr Sandbach claims costs have spiralled and The Shrewsbury and Telford Hospital NHS Trust (SaTH) is now looking at options over whether to scale back the scheme.
Dr Julian Povey, chair of Shropshire Clinical Commissioning Group, told the organisation's AGM on Wednesday that SaTH was drawing up the business case and he had not seen any calculations by the trust.
He said: "We are not sure where this figure has come from. As far as I'm aware the money we have been allocated is £312m.
"Whatever is provided has to be in line with the model we've consulted on."
He said SaTH was drawing up the business case and had a deadline of October.
Dr Povey added: "They have shared no figures with us."
Mr Sandbach claims a document prepared for the sustainable services program steering group last month revealed the cost of delivering Future Fit has escalated to around £420m.
It comes as Health Secretary Matt Hancock is yet to decide whether to review the decision by health bosses to have a single emergency centre for the county based in Shrewsbury – which could ultimately place the scheme in jeopardy.
The decision on Future Fit was made by a joint committee of Shropshire and Telford & Wrekin CCGs.
If the scheme is allowed to move ahead, Shrewsbury will become the base for the county's only emergency centre and PRH would also lose its consultant-led women and children's services.
It would mean PRH would take over responsibility for planned care in the future.
Both PRH and Royal Shrewsbury Hospital would have 24/7 urgent care centres, but the most seriously ill patients would be taken to Shrewsbury.
An independent panel, tasked with deciding if the decision should be reviewed, has been interviewing hospital staff, bosses and health commissioners, and will be meeting with council bosses next week.
Recommendations will be made to Mr Hancock who will then decide if the review is necessary.