You're not the only one who feels ripped off. A tip I heard from someone in a local community who gotten 5+ invites is to not buy any coins at all. Limit yourself to 1 free raid pass per day.

Niantic is just like any other gacha mobile games out there. When you're a big spender, they lower your chances on the lottery system. When you stop spending, they increase your chances to entice you. If you played many of those gacha games, you'll notice the similarities. That's why Applestore made a new guideline recently last month to make it requirement for all gaming apps to disclose the odds of "lootboxes" in gacha games. This means this is a real thing that's happening in most if not all of those gacha games. There're many games out there who would do such things. Too bad Ex Raids aren't considered "loot boxes" (but an indirect way of gambling), so they're not affected by this new requirement by applestore.