You will be hard pressed to find any serious miner who is keeping mined Doge instead of converting them to BTC or fiat. Serious mining farms pay for their electricity, thus they will not have amortized their GPUs solely from Doge.
Sure, if you are a gamer with "pre-paid" graphics card AND "free" electricity, you may elect to keep your mined Doge.
Blocks per day: 60 * 24 = 1440 (https://bitcointalk.org/index.php?topic=361813.0)
Current block reward: ~500,000 doge (https://bitcointalk.org/index.php?topic=361813.0)
Total block rewards per day: 1440 * 500,000 = 720,000,000 doge
Current exchange rate: 1 doge = US$0.0017 (http://dogecoin.com/)
Total block rewards in USD per day: 720,000,000 * 0.0017 = $1,224,000