If in fact it is gone, it's a misdiagnosis. There's nothing you can feed your rat or do at home that will cure a tumor, no matter what you may read on the internet... although I certainly understand trying whatever you can while saving the money for the surgery. The rate of growth of a tumor is simply too great for any natural processes to work on, which is why surgery is the only cure.

That said, preventative measures (for any future tumours, if this was indeed a misdiagnosis) may be another story, and if your rat enjoys the broccoli then by all means continue on... I've also read flax seed is good for preventative measures, and is good for the kidneys too... but be prepared to clean the cage more since whew will their poo smell more!

Do be vigilant in feeling for the tumor however, to be sure that it is gone, and isn't just hiding from you. I just went through the same thing, but with the opposite effect: the tumor grew like bloody crazy, so it was immediate surgery or lose my buddy pretty fast. You'd be amazed at how well these little guys pull through surgery... if you're interested, my post is a few down, with pics of the recovery and all... "Boober's Tumor".

So, fingers crossed for a misdiagnosis!

As for cost... Boobers' tumor removal cost $270 CDN, not including the initial visit and the optional biopsy, which is about 133 pounds... so your vet seems to be in the same ballpark. (plus, everything costs more on your side of the pond!)

 

I agree with Schmea...

It was either an abscess and it drained on it's own or it was a fatty deposit which sometimes will just shrink but may come back.
I've heard the drug lupron might be working in shrinking tumors and preventing them.
Also spaying your girls reduces the risk of benign mammary tumors on females. It has worked wonders for my gang.

Broccoli is an excellent treat/supplement for the rats, I would continue to feed it to them.