Anchoring is a very common tactic that retailers use to sway people towards making more expensive purchases.
1) Anchoring - By placing a cheap item next to an expensive item, the cheap items looks more affordable. For example, if you can get a bronze item for $1 and a silver item for $5, many people may purchase the cheapest item. However, if you add a gold item for $50, the middle option (silver) now looks like a good deal and more people may purchase it.
2) False Hierarchy - A game may use layout, size or color to give a more expensive item preference over a cheaper item when in fact the items should be compared side by side.
3) Decoy Effect - Using a decoy item, that is usually more expensive or inferior, to push people towards a specific option. For example, items A and B are available for $1 and $2, but a decoy item C is $3. Item C is inferior to B. This makes B look like a really good deal since you are getting something worth $3 for only $2. Item A becomes less desirable.
4) Discounts for buying bundles or bigger packages of items incentivize people to spend more to get a better deal.
5) Social Proof is when a store promotes an item in way to suggest that more people favor it. This could be as simple as having a "Most Popular" badge on top of the item, or it could say something to the effect of "15 people have purchased this in the last hour". If the game integrates with social media, it could also use your friends
to push you towards a more expensive purchase. For example, "John purchased a Gold upgrade today". This normalize spending on the game and makes it feel more acceptable.