You are not going to find a sabotted slug for much less than $2 each.
This is assuming that you are putting a scope on it. If you want to save money while sighting it in, try the fiochi dangerous game slug and once you have it the best you can get with them then move up to the sabotted slug. Fiochi's are around $8 for a box of 10.
Keep in mind that your slug gun will be finicky on which speed/weight of slug it likes best. You will need to go buy several boxes of different speeds and weights to find out which it likes best. There is no cheap way around it if you really want to know what is going to perform the best in your slug gun. You can't just say, I like the performance of the hornadays so I am going to use them and expect to get good results. Let your gun tell you what it likes best.
When I was trying to determine what worked best out of my 870 with a hastings barrel, I purchased a box of 3 inch lightfield commanders, 2 3/4 lightfield hybrids, 2 3/4 hastings magnums, and 3 inch hornadays. At 50 yds, the commanders and hornadays were all over the place. The hybrids were grouping at about 1 inch and the hastings were ripping one big hole. I bought more hastings and shot at 100 yds and was getting 2 inch groups, sometimes better.