The simple answer is often the right one.
The manufacturers have spent exhausting hours developing these slugs and ya gotta know that there are a number of competing companies that are all pushing each other to make fantastic products.
It's simply not ever going to be as easy to just lop the end off a cheap slug and get the best results. Most likely--you won't even get good results, and the pitfalls that await make the whole idea seem fruitless, in my opinion.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.