Some lenses are build splashproof, while others aren't.
What is the least expensive method to make a lens splashproof which is not splashproof?
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It depends on what you mean by splashproof.
Rain covers are the absolute cheapest form of protection (as little as $5 or as much as $70+), though they generally leave the front element exposed and provide only minimal protection since they are not sealed. They also are somewhat clunky to use as you generally have to reach under them or through sleeves in order to get to controls and you have to look through a plastic window to see the camera controls.
The next level up is water tight bags. These cost more than rain covers on average ($60-$300), but they actually form a water tight seal around the camera. This comes with a down side however. With no way for water to get in, there is also no way for your hands to get in and you must try to operate the camera through the bag. The objective is also covered, so it does reduce image quality, though models are available that provide an optical quality windows for the objective (though they cost more). They are also limited to only a few inches to a few feet of water before the protection breaks down, though if you only care about dust and splash, they should do the job fairly well.
The next level up are sports enclosures ($350 to $1000). These are similar to underwater enclosures, but without some of the over-engineering that goes in to making it withstand depth. They are designed to handle similar conditions to the watertight bags, but are far easier to use and almost always have optical grade objective covers. They are bulky, but they allow easy access to controls and are far more reliable in terms of simply placing the camera in to it and closing and latching the door.
Finally, the absolute best protection is the full blown underwater enclosure. These units are extremely expensive ($1000 - $7000) and have to be assembled to a configuration specific to the lens and body being used (modular systems allow you to re-purpose it for different lenses by changing out parts of the enclosure). They offer dedicated physical controls and optical grade objective covers. They are engineered for even deep underwater and can often go 300 feet down or more. They are probably overkill on land, but they will withstand just about anything you can throw at them without severely impacting your ability to shoot since they offer dedicated physical controls. They are, however, not available for longer lenses.
Of course, the list wouldn't be complete without suggesting simply a weather sealed camera body and lens combination which would preclude most of this stuff being necessary. Note that a weather sealed lens alone is not enough though and weather sealing. The entire camera system (lens, body and battery grip if applicable) needs to be weather sealed and it is only as well sealed as the weakest component. Also, while professional weather sealing will handle light to moderate rain, it may not hold up to the worst of conditions. I've done shots with my 5D Mark iii and 70-200 f/2.8 IS II in moderate to heavy rain, but had to promptly deal with some minor moisture issues after. I suppose the cost of getting a 5D Mark iii and a 70-200 f/2.8 IS II is actually more than getting even a cheaper underwater enclosure, however you also get substantially better image quality from it.