If you run hard aground it is Salvage. The same is true if the have to use a pump if your taking on water. My understanding is salvage cost can be up to 1/3 of the boats value. As stated last week in some ones thread before they tow you ask if they consider it Tow or slavage.
Here's part of a web page on Boat/US's site with their definition. As for cost, the idea of salvage is that the salvors are saving your boat from peril, that if it were left alone, it would eventually be a total loss. They are also risking their boats and equipment to save your boat. By doing that, they are saving you (or most likely your insurance company) from losing your boat completely. While it seems absurd that they could charge your 5-30% of the value of your boat, your insurance would rather pay $10,000 to them to save the boat rather than $100,000 for the total loss. Anyway, here's what they have to say:
To be an approved TowBoatU.S. operator, marine assistance companies must agree that Towing/Ungrounding is any operation not involving immediate danger to the boat or to a legally protected marine environment. It requires just one towing vessel with lines attached to a grounded boat to refloat it or to the disabled boat to tow it. If a grounded boat can rest without peril until the tide returns to float her free, or a boat is drifting in calm conditions after losing power, it almost always calls for towing, not salvage.
Salvage, on the other hand, involves imminent peril to a grounded, sinking or stranded boat or to a protected marine environment, or the use of more than one towing vessel and/or special salvage equipment such as air bags or high capacity pumps.