GPT-4, especially with GPTs, is computation-intense. When paying on the API for the actual data services, the increase between 3.5 and 4 is 10x the price.
$20 were it even for 40 questions per day in each of 30 days is a bargain compared to API costs - and you get that many times per day.
Perhaps something like 60 per 6 hours might align better with actual patterns of use, while still keeping off abuse of computation resources.