CityMaps2Go (Ulmon) is having a sale this weekend. Their Premium service is only $9.99 (discounted $20).
The App is free, but you only get one map. If you upgrade, you'll get Unlimited Offline maps, no ads, and you can customize the maps and lists with colors, shapes and pins.
If you miss the sale this weekend, it seems as if they have sales around holidays and long weekends. So you can always catch the next upcoming sale.