About how many DQ items would it take to get 25 Silver Orders?

each Dragon Quest hast 10 sub-quests.. Mostly 5-6 DQ Parts.. So lets assume 6 for now.. I think it was 10-15 DQ Items, depending on lvl. So lets pick the higher number again.

6x15 = 90 DQ Items for one chain(and these DQ Items have to be of a spetial kind).

lets assume you need to buy them all. Than you can only spend 15k per DQ Item so that you are still below the 1.5kk.
For 25 DQ Items you need to repeat that. If you are still not high in lvl you will have the chance that you only get bronze, which means all the DQ Items don't count. On higher lvl you will have the chance to get gold which is nice, but also not what you want.

So all in all you need 30-40 Runs of DQ if you can take the option that the DQ Quest gives gold/silver badge. If you only can take the lower DQs liek 31-61 you will have to run much more runs.

My conclusuion is: If you want that Orders, buy them. It will take you a real long time farming the DQ Items or much more coins to get the DQ-Quests done.

On my first toon, a veno, I did making silver dragon orders for money. Let's just say it was really painful, and very very tiring. Plus, there's a 10% chance of getting a gold instead of a silver, which is worth close to nothing.

I'd suggest just grinding and finding other ways of making money, and using the profits to purchase the orders. It's a lot easier and faster, and you don't feel annoyed when DQ items don't drop when you're grinding.

one more thing to add to beforesun's analysis:
while you can afk fly to the DQ npcs you still need to spend some time...
and from my experience, afk flying is not so afk; I check a lot of times if I have reached the destination and cannot really concentrate to other tasks.

so, while it's not pure game time, it still takes some of your time.
time you could use to farm or grind and make even more money

btw on DW i rarely see DO for sale