It looks from the pictures that the sill is beyond patching and renewal is the only option. The structural integrity of the vehicle is compromised with this level of corrosion and nothing other than a comprehensive repair is acceptable. Covering over with patches will not help in an emergency if the seat belt anchorage is not completely supported. Costs for a repair such as this are very difficult to quantify as the full extent of the repair cannot be seen until the old sill has been removed, and once removed you are committed to carry on. It appears as you have time constraints, so either lay the vehicle up until such times as it can be repaired or bite the bullet and get it repaired properly now, remembering that repairs such as this just eat money and if someone offers to do it for a few cans of John Smiths they are probably using the empties for the patches.
This may be not what you wanted to hear but as you are asking for advice from forum members all comments should be considered.
