Google Wallet has been a hit to Google's metaphorical wallet. According to Bloomberg, Google has spent $300 million in acquiring payment startups to bolster its digital wallet product. This on top of internal resources dedicated to making Google Wallet a success.

While Google didn't offer any information on how much it has spent on the Google Wallet effort, Bloomberg did speak with Google Wallet's former chief Osama Bedier. According to Bedier, he was encouraged to spend freely to develop Google Wallet during his tenure.

If Google and Isis, the carriers' mobile payment effort, are any indication, building a mobile wallet isn't for the faint of heart or cash-strapped. Isis has reportedly spent at least $250 million on its NFC-enabled mobile payment application, with a source familiar with Isis' efforts putting that number closer to the $350 million range.

And all that money certainly doesn't guarantee adoption. To this point, Google Wallet use, at least for in-store purchases, has remained modest at best and the app is only available on certain phones on Sprint's network. The app is effectively blocked by Verizon, AT&T and T-Mobile — the partners in Isis.

As for Isis, it's still in trials in Austin, Texas and Salt Lake City with an expected nationwide rollout sometime this summer. 

Read more about digital/mobile wallets.