Republique went the other way - they made it for mobile first and then ported to PC. They also planned for this and used a game engine that already supported those platforms.
If this game is using a PC-only game engine, then I'd optimistically guess you'd need an engine programmer to sped roughly two man-months per engine "system" to port it - or easily a year of work. That alone could cost $80k-$150k.
If it's on a portable engine, then you'll probably want to first port the game to the latest version of that engine and then just "build" for your target platforms. That's probably a few months of bugfixing.
Then you've got performance issues. If the original gameplay doesn't work on your target hardware due to perf problems, you may have to scale it back, which requires thr hands of a gameplay programmer and designer.
Then there's content issues - the actual 3D artwork may not be suitable for phones so may need re-authoring. That could be months of art time.
Then there's the renderer - the same rendering techniques might not work so well on mobile, so you could need a graphics programmer to rewrite their pipeline, shaders and data processing tools...
And the networking - if this is a multiplayer game originally designed for DSL and you want it to work on 3G/4G, that will surely warrant a rewrite...
So at a guess, cost would optimistically be anywhere from ten grand to around a quarter million or even a half million (or more depending on how much stuff is incompatible).
If you know what the original budget of the game was, it probably won't cost more than that...
What you mean with "portable engine"?
Already for mobile or with crossplatform engine ready?
I was able to play Half-Life 1 the real version of PC and many compatible pc version mods.
The only thing i had to do was to download an Android App "Xash3D" on Android.Than i create a single folder named xash on my sdcard root and just copy inside the original "valve" folder from my PC of the game Half-Life 1 directly purchased with my Steam account.
And obviously mobile touch controls are not that good cause the game was originally designed for PC but if i play with a professional bluetooth gamepad
the experience is just amazing and is working smoothly also with midrange androids phones.Plus i'm able to play in some multiplayer rooms.Obviusly with a decent WiFi connection...
The only thing is not really plug and play for most of users.They might not even realize what the app xash3d is made for.And even if they understand is hard for them to understand how it work with all the passage of trasferring data from PC to mobile...Than need to set all the parameters to make it work properly with a gamepad bluetooth.
But at the end the result is "excellent" is was really able to play Half-Life 1 PC full version on an Android smartphone/Tablet.
Let's use this sample Xash3d is not affiliated in any way with VALVE developer of the famous Half-Life saga.They just create this app to run the game on mobile using the real PC folders.I guess is a sort of engine emulator.
But let's say hypothetically Valve corporation provide me the source code allowing me to create a full APK wich is not only an empty app but include directly the game files of their old Half Life 1 i can understand how the experience could be worst than now with Xash3D wich is already pretty perfect.Don't think a 1999 old PC game couldn't run smoothly as Half-Life do on most of nowadays midrange mobiles trought Xash3D.
To create an APK of Half-Life 1 or similar ported on Android i need that much money close to half million usd?
Do you think the russian developer of Xash3D spent that much?
Cause i mean he didn't include the file in the APK just for a matter of agreement of with Valve corporation but if it was allow to do that was really easy for him include the files directly with Xash3D downolad...99,99% of the job was the realization of Xash3D app certainly not copy and paste existing Half-Life 1 PC folder he don't even need to change a single original file to make it work on mobile opening with Xash3D...
Ok is possible Valve Corporation would never give any authorization to use their PC files they are still selling on Steam for 9.99 usd just for PC.
But i'm trying to understand if what the russian developer did with Xash3D is possible to replicate with others less famous "old PC games" getting the same excellent smooth result on latest mobile devices...
Is hard to me to create Xash3D wich is totally free and only 50000 downloads the russian developers had to invest the money you told me close to half million dollars.Or at least would be crazy with no sense to do that or not?