I have a 3 ton heli natural gas pack and it is bad I have a friend that has a 3 ton prpane gas unit that he will let me have can you convert that unit to nautral gas and is it very expenive

It depends upon the exact items that will need to be changed. On a conversion it is always better to have a HVAC professional do the job so I would contact one of your local companys and get a estimate. Different prices are charged in different areas of the country so there is no way to give you a price.