Which will be the best and cheapest way to increase the database of an ecommerce platform?

By "increase," it sounds like the data base already exists. If you want to add more items to the data but have no need to increase the amount of data on each item, that should be fairly easy. There might be provisions in the system now for doing that.

If you want to increase the amount of data on each item, that makes it a lot more complicated because the use and display of the added data would have to be written also.

Assuming there is no provision built into the existing system, to do it either way, you would have to have access to the source code and the skills to change it. The ease and the cost would depend on what you have now, whether you can do it or you have to hire someone else and what they might charge. In that case, you just have to find that person and get some quotes. "The devil is always in the details." :-)