Can I use a combination of extension tubes + 50mm prime + raynox dcr 250, for a high magnification macro setup?

The numbering of the Raynox lenses is in terms of "magnification" in reference to some apocryphic camera rather than in terms of focal length. Usually a number of 250 in a closeup lens implies a focal length of 250mm (and thus 4 dioptres since dioptres are the inverse of the focusing distance in meter), but the DCR-250 has 8 dioptres and the DCR-150 has about 4.8 I think.

8 dioptres imply a maximum focusing distance of 125mm, 4 dioptres of 250mm. Those Raynox closeups have lens threads of 43mm and front threads of 49mm. They usually come with a clip-on adapter fitting lens threads from 52mm to 67mm. With an active diameter of 43mm, they can vignette on lenses with a comparatively wide opening.

Now "maximum focusing distance" means that a focus distance of ∞ of your normal lens will get mapped to 125mm of focusing distance with the DCR-250 attached. However, if you used extension tubes on your lens, you no longer have a maximum focusing distance of ∞. Extension tubes may already reduce the focusing distance to a degree where the added 8 dioptres is a drop in the bucket: note that closeup lenses with a single group do not add any noteworthy magnification of their own but merely reduce the focusing distance. While the DCR-250 is a 3-element lens, its 0-distance magnification is not much different from 1:1.

So with extension tubes already reducing the focusing distance, the tradeoff between lens errors and additional magnification may not be worth the trouble.

Where closeup lenses make a large difference is when the minimum focusing distance would otherwise be large. This tends to be the case with tele lenses, or the long end of zoom lenses.

I never heard of the Raynox DCR 250 before so I Googled and read their advertisement. This type of advertisement makes me raise an eyebrow, meaning I view what they say as “puffing”.

This lens accessory, in the vernacular of the photo industry is a “close-up” lens. These are supplemental lens attachments in common usage. We mount them by screw thread or clamp in front of our ordinary camera lens. When mounted they allow the camera to camera to work in closer than the camera’s normal close focus lamination.

These supplemental lens resemble reading eyeglasses and as such their power is written is a unit of measure common to the reading glass vehicular which is the diopter unit, The DCR 250 is a cipher for 2.5 diopter. We can convert this unit to millimeters via 1/2.5 X 1000 = 400. This tells us that if a 2.5d supplemental lens is mounted, the camera when set to infinity (∞) will focus on an object 400mm forward of the front lens = 15 ¾ inches. Bottom line, you can focus closer and thus gain some magnification but this higher magnification is not spectacular. Likely your camera, without mounting this close-up lens can match this feat.

Yes, you can also mount extension tubes and make this a more complex lash-up. Keep in mind, supplemental close-up lenses induce aberrations. The chief aberration is chromatic, a color fringing surrounding the outline of objects. The Raynox DCR 250 is advertised as a multi-element supplemental thus it likely will do an OK job.

Been there, done that(*).

My experiments in a distant past showed me that the close-up lenses would work up to some focal length, beyond which the picture was starting to get really soft. So on a 50mm you should be fine. But the DCR-250 may bot be as wide as your front lens and therefore cause vignetting.

Will the result be practical in the field? If "the field" is a place where you intend to shoot moving subjects such as insects (or even flowers lightly blown by the wind), then the answer is "not really". Your problem is that both the DCR-250 and the tubes make you camera near-sighted. With the DCR-250 (8 diopters) your camera (and you, looking in the viewfinder) cannot see anything farther than 12.5cm (5"), so just getting your subject in the frame can prove difficult, because you cannot step back to widen the frame. Adding tubes will only makes this worse.

The other problem is that not only the depth of field is very thin but your AF system is going to show its (speed) limits. With the lens and tubes, a slight longitudinal move is akin to a subject moving several meters with the naked lens. And you'll need the AF, unless you get a lot of training doing manual focus without changing the subject-to-lens distance by more than half a millimeter.

So unless you have a perfectly still subject and a perfectly still camera the whole set up is going to be hard to use, expect a lucky shot for a couple hundred photos.

And if you have a perfectly still subject and a perfectly still camera this isn't "the field" but "the lab" (or "the studio").

So, my advice: start small, with a thin tube.

(*) Had a DCR-250 on a bridge camera, and then used the Canon 500D (2 diopters) and 250D (4 diopters) close-up lenses on a 55-250mm zoom. I got a few miracle shots with the 250D but the 500D was a lot easier to use.