There are many paths to consider in cases such as these and the cost of making your own solution will come down to the type of Colloidal Silver you're looking to produce. ie, The least expensive method would be that of room temp Ionic Silver made in smaller batches(.5 - 1L). Whereas the most expensive would be that of a combination or elemental silver solution, which would likely involve a heat stirrer and a few other specialized items.

That being said, many people do quite well with the basic Ionic solutions, which can be made very easily with only a few components that can be purchased on eBay and/or your local electronic store.