Mafia City - lvl 15, spent $8, got $30

Offerwall: Adgem (android) 

Reward: 31,320 coins. Credits at level 15. 

According to another fc user it is both player/boss level 15 AND mansion 15.

I spent $8 total. It took me 2 days (I was broke so I dragged this out some). I got to level 12 with three .99 cent packs, then sped it up more the next day with a 4.99 pack, which in my case met requirements for the Newbie Purchase Benefits (lot of speedups) in the Event Center. The newbie rewards + 4.99 pack carried me to 15. 

Exact packs:
Apparently the exact newbie purchase benefits event can change, so look at yours first 'cause you might not need all these.

.99 Family Uprising Rookie Pack (2000 gold, lot of speedups and resources)
***.99 Valuable Pack (500 gold)
***.99 VIP3 exclusive pack - evidently I upgraded to VIP3 at some point. It was the Natalie fragment pack, 1000 gold and a few speedups.
4.99 - Family Uprising Rookie Pack - 4000 gold, tons of resources and speedups

***These two didn't have much, I bought them specifically to meet the newbie purchase requirements.

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If you're not spending, I think it's still pretty doable but I'm not sure exactly how long it'll take. Before you hit 6, get a ref code from someone to put in the treasure box by your mansion. You get rewards as you level up. Your clan is really important and once you level up, make sure you're in an active one. Clan jobs, kickbacks, helps, donations, all that = speedups. Also check the garden out. 

Usually MC clan leaders are really helpful to newbies in my experience, so pay attention to chat and any clan leader messages you get.