MM,
You should be able to find an older Whites Goldmaster (black case model and not the very old blue units) in the price range you mentioned if you are patient. In fact, I sold one for $200 a while back. Patience is the key. Watch Craigslist also. Heck, I even bought a very rough GMT for $250. It is rough but works well and for me, that is all that counts. Being pretty or without marks or dents doesn't make it work better.
Detectors are tough. In fact, many years ago I loaned a friend a couple of detectors and wouldn't you know it, his wife ran over them deliberately. She didn't know she managed to hit mine also. Well, they were twisted and bent but still worked fine. The friend's detector got the worse of it and ended up needing a new case, but still worked just fine. Worse case is I Loaned a friend in AZ a ML VLF gold machine. I think it was an XT17000. Well, he managed to roll his truck and the detector took an extended air borne trip a couple of hundred feet past where the truck stopped. Bent and bruised, it still worked just fine. So, they are tougher than people realize.
Don't be afraid of buying used. Most US manufacturers will repair their units for a reasonable fee if they do fail. Fortunately, I own a bunch and have owned a lot more and only had a couple of detectors fail out of the 60+ I have owned over the years.
Right now I have a GM 4 and a Minelab XT 18000 I am thinking about selling but it would probably be a little more than what you are willing to pay, but let me think about it. Both work well as does an older Lobo (I might sell) I modified that also kicks butt. All three will find gold with the GM 4 finding the smallest. The XT 18000 is the easiest to use because of the auto ground balancing. However, the old Lobo has been modified and will double as a coin hunting detector also.
Changing the subject, are you saying someone found a 4 gram nugget at Point Bar

? If that is where the guy claims to have found the nugget, I suspect he was joshing you. There is a small chance something like that could happen up at Cash Creek because I know some larger gold has been found up there, but I never heard of any found at Point Bar. That location is known for its super small gold.
There is a prospecting club in Florence if you are interested. I don't know much about it, but it is close to you. Maybe some of the club members have detectors and will show them to you.
BTW, I live down the road from you in Pueblo. Email me if you have any questions, especially before buying a detector. I have been around them for a very long time and have owned a large number of gold detectors. I still own quite a few right now. So, I should be able to give you an opinion. I might even own one of them or something similar you could try to see what you think.
My detectors range from very old to some of the latest. Heck, I was fooling around with a very old Gardiner 202A the other day. This detector was built in the 70's and was used for gold hunting during its time. The other day I did a little testing and was really surprised that this old of a detector would ground balance very well for its time and detect a 2 grain nugget I use as one of my test pieces out to about 2+ inches. Now, I don't recommend any detector that old but it is surprising just what works reasonably well and what doesn't.
Reg