I would buy them at a rate of 25 DMD / .15 BTC
That price is comparative to what it would take to mine something like LTC vs DMD.
Good luck to find seller at that price
Just saying your .1 BTC is completely unrealistic for current development status.
This is rare type coin, it is for investment, like gold, silver, diamonds. Therefore it's price is not determined based on usual stuff like how many gambling sites accept it or if it will be possible to make small cheap transactions. It's value is determined on how hard it will be to mine and how small amount of total coins there is, just like gold, silver and diamonds. Currently you have offer for 0.1 BTC, after year you will pay 5 BTC because then value will be determined by those aspects. And those say - there will be 5x less DMD coins that there will ever be BTC coins, and mining DMD is and will be much harder than BTC.
Don't get me wrong, I don't want to scam you. If you don't want to play on long term (1+ year) investment then you shouldn't buy DMD for this price or for any price as in short term you will not see much gain from it.
same as you shouldn't buy gold if you don't want to invest for 10+ years, because in shorter term bank account for gold storage costs more than profit you make from gold.
Be aware, as any investment, long term investments also are risky, you go all or nothing. As you maybe have heard, current gold prices in real world are crashing like never before ... and noone can predict if they will ever go up again.
EDIT: And yes you shouldn't agree on mine or any other persons opinion, but think for yourself. ALWAYS, not only with DMD but with any coin.