".357Magnun" and "cheap" or "inexpensive" are mutually exclusive terms. If you want to shoot a lot of .357Mag for cheap then you need to get into reloading.
Once you've got the brass it literally is a question of about 1 to 1.5 cents worth of powder between a .38Spl reload and a .357Mag reload. And even using bought bullets reloads only cost around $5 to $6 for 50.
If you shoot much and do the math you'll see that the cost of the reloading equipment will be amortized out within a few thousand reloads compared to buying factory ammo. And even if you live in an apartment and can't leave an area set up for reloading it's still possible to set up for a modest setup that all fits into a single storage tote and goes back into the closet.
For factory ammo anything much under $30 seems to be a pretty decent price. But it doesn't help if you have to pay for shipping on top of that.