You can easily figure out your profits on the forge. It is about a 1/16th chance of getting a better item. So every 16 combines you should get a better version. Every 4 failed attempts results in a 5th attempt from the created materials. So to get 16 tries you need 13 attempts (+3 attempts from failures and one successful upgrade). Take the cost of combining and multiply by 13 to get the cost to create a combined item. Now multiply the price of the upgraded item by .85. If the sell price of an upgraded item is greater than the cost to create it you will make money. To see if you will usually win in this process, take the cheapest item you can create when combining four components and see if the math works out for that situation.
Things get more complicated when there are multiple options, some resulting in profit and others in failure. You need to do more math in these situations and look for the % chance of "winning" versus losing money. It's like playing a slot machine except now you know how to pick the one with the best odds.