An attempt at valuation:

Bitcoin - 12.5 coins per block, 6 blocks per hour = 1800 coins per day x $3600 = $6,480,000 cash injection required per day to maintain current price

Grin - 50 coins per block, 60 blocks per hour = 72,000 coins per day x $6.5 = $468,000 cash injection per day to maintain current price.

Therefore Grin requires 7.2% of the cash injection of Bitcoin on a daily basis to maintain its current price.  

Is Grin worth 7.2% of Bitcoin at this time?  Definitely not.  It is an untested coin with extremely low adoption and a serious pain in the ass to transfer as both parties have to cooperate to make a transfer happen.

Conclusion:  Grin is currently highly overvalued at $6.5