It ultimately depends on the game itself & the content provided. On premium games, I have spend up to $20 on a single standalone game & over $100 on a single game with premium, non-consumable DLC. I will note that the games I spent money on were well known games by respectable AAA publishers that have high replay values, so I'm getting my money's worth on them.
Those that were more indie, I was a bit more hesitant in opening my wallet... especially games that their microtransactions & DLC (especially the consumable DLC) are nothing more than cash-grabs. However, with some mobile sales (like those on Humble Bundle), I'm a bit more willing to experiment since I'm getting a reasonable deal.
If a new game came out that's $5 in the Google Play Store (as I'm not into iPhones), I would dig into the details & possibly look at some outside sources if it seriously interest me before pulling the trigger... so the makers of the game would have to do a fairly reasonable job to earn my business, as I'm not afraid to request a refund if things don't work out within the store's window (which I've only done with a handful of games due to technical issues).