It depends on the situation and region, but in more cases than not digital is a steal compaired to physical. 

In some situations 1 copy of a digital title will give you access to a game on multiple devices, as opposed to buying multiple copies.

With online games, you get two games for the price of one. My Pro in the main room is my primary unit, so anyone can use it to play online, I can then use either my office or bedroom units and play with them using a single copy. That is a huge value for families.

In the US most games go on sale on PSN extreamly quick. I have picked up many games at half price within a month of release. At this point if I don't have to have a game day one, I just wait a few weeks and grab it on sale.

Then you have PS Now and Xbox Game Pass, you can get hundereds of titles a month for $10 - $20.

There are many ways that digital is cheaper and have greater value. At this point the only reason I see to buy physical where I live is if you are a games collector. Even if you have slow internet there are better value services if you are looking for short term commitment with games than buying and reselling.

its usually the same price. and with digital, you can share with friends

It's the same team, making the same salary. The programmers who made the 3DS versions didn't sudden make less than the PS4 people (that's assuming that the team was even split that way between PS4/3DS). Without knowing how the work was split up and how Square accounts for cost in a development situation like this, there's no way to definitely know how much either cost.

PS4 games will cost more than a 3DS game because the larger team needed in order to make a big game. But DQXI isn't a normal 3DS game, it had a giant team and giant budget too. If Final Fantasy IX can cost $40m to develop on the PS1, there's no reason why a 3DS game can't be just as big budgetwise. While it's a safe assumption to think the PS4 version cost more to develop, there's no evidence to suggest that there's a large difference. Especially so when the CEO of the company is praising that specific version.

Dude, are you serious right now? You really think the 3DS version cost as much as the PS4 version to make? It doesn't even look good for a 3DS game (only the retro mode looks good). 

Ever Oasis looks much better graphically and that was made by a small Japanese third party developer. 

And why on earth are you comparing budgets from PS1 days with budgets of today? Developing games for old hardware like the 3DS is a much easier process than it was 17 years ago when that technology was still cutting edge. Development tools and programs change over time as well you know. 

No. Did you read my post? I said there's no reason to think that there's a large difference in cost between the two versions of the game, because so much of the work is shared (writing, music, design, art, etc are both completely shared costs).

How good something looks really doesn't have a direct coorelation to cost of development. It's the size of the team that leads to high costs. And of course development tools and things progress, but this is still a 3DS game that had 100+ people working on it for over 3 years. It's budget would still be in tens of millions without question. A game doesn't become cheaper due to a platform. It's due to cost of supporting the team making the game.

And you apparently know how long they worked on the 3DS game and with how many people?

The games are nothing alike besides a shared story. I seriously doubt 100 developers were working on the 3DS game for 3 years. The 3DS game probably started development later because the director of the game said the 3DS version was an after-thought and it was originally going to be a PS4 exclusive. 

As for the music, it sounds horrible on both the PS4 and 3DS. 

Well, a portable console version.