The issue is that minimum resale pricing is set by the importer, NikonUSA in this case, for all major camera brands. Compliance is required by the dealer agreement. A price offered by one dealer which is out of step with what other dealers are offering is a major red flag. This may be a camera without a full complement of accessories, a camera with documentation in another language, or with a battery charger not compatible with US standards. It is almost certainly a camera with a serial number which makes it ineligible for registration under US warranty protection. Nikon will also not even do paid service or repair on these cameras.

42nd Street is advertising warranty protection for this camera at prices up to $300 or more. That is further indication that this is likely an improperly impirted camera. If none of this is a problem for you, then go ahead. But do so knowing the risk you are assuming to get the low price. On the other hand, buying the warranty service or missing accessories is going to cut deeply into those expected savings. A Nikon charger for this camera (which would ordinarily be included) costs about $70. If a battery is what is missing, the cost for one like what should have been included is $60.

"Not see a difference" refers to image quality. Theres some small IQ edge to my 750 over my 610. "Small" means if you dont carefully look for some quality difference youll never notice any.

I dont put the two cameras into a controlled "A-B" comparision, but the 750 does seem to focus with very few small errors and the 610 seems a bit more error prone.

I routinely use very high ISO so noise is just normal to me. When youve got plenty of noise its not esspecially noticeable if one camera has just a bit more of it than another.

I *SHOULD* say something like "I pity fussy users who endlessly test image quality". I *should* say that but I dont cuz it makes me laff so hard that coffee shoots outa my nose ... okay, slight exageration.

•••••••••••••••••••••••••••••••••
•••••••••••••••••••••••••••••••••

A "better" camera is all about better enabling the user. The IQ difference, if any, is NOT what makes better pix, its the "unshackling" of the user. Im all for better cameras. *I* can feel the user benefits. But if your 610 doesnt make *you* feel shackled then keep it. A 750 wont noticeably up your IQ and altho Ive never touched a 780 I have never read that its a real jump up imagewise from a 750.

If you want a more enabling camera, SLR deveopment has ended. The 610 is a "fairly recent" midlevel SLR. Youll hafta go beyond SLRs to find a truly more enabling camera ... and the IQ upgrade still wont seem really impressive. But the upgrade WILL be there if you love running controlled tests.

FWIW I use 610, 750, Z6 and Z7. Acoarst the Zs now get nearly all the action, but I do have solid user experience with the two SLRs. Its just that Ive moved on. As I said, I do LIKE better cameras.

Noise performance is not linear, not across ISO values for one specific camera model, nor across ISO values from different models.

Higher available ISO maybe worthless, or very usable. It depends on the camera, the exposure situation, and the photographer's needs. There are no applicable and reliable generalizations.

Again, seeing an actual result from the D610 would be the appropriate basis for discussion of the current issues and the relevant causes and possible corrective actions. It would be sad, but not unusual, to throw $1300 into another camera to achieve no tangible difference.

Instant focus? Probably not. The AF system is likely to be limited by (a) the focus motor of the lens and (b) the power supply of the camera. Typically, a battery pack makes a different to the highest end cameras in giving 'more power' to the camera. Changing the lens changes the lens focus motor. Battery packs cost another $100 to $300, based on the brand / model involved.

Faster speed? The reference to frames per second (fps) seems to be different than your usage of 'faster speed'. The D610 has a shutter speed limit of 1/4000 sec. The D780 provides 1/8000 sec max speed. Is shutter speed your meaning of faster speed, and is faster than 1/4000 sec relevant to your needs?

FPS? You have to be shooting still photos in Live View to achieve 12 fps for the D780, including a number of caveats on the file type and card type / model. Is LiveView even practical for sports, wildlife, etc? Otherwise, you're at 7 fps that doesn't seem like much for $1300 for just another 24MP Nikon full-frame DSLR.

reliable faster AF? Have you presented examples images and asked for help, aka free help? We can probably save you $1300 by just discussing your current D610 results. Maybe there is a need for another camera for your needs, where a D780 isn't the solution. But, we don't yet know the problem.