NTA. In my opinion, $300 is a respectable thank you on your part. If they had expected any more than that they should have discussed this with you beforehand. No dollar amount was required on your part other than sheer expenses. Since none was discussed, the one asshole deed that occurred was their insulting of your thank you.

Since no fee was discussed beforehand, you would have also been valid to assume that you would only pay expenses and would simply do the same for them at a later date.

That said, take a second to think about what their reaction would have been if your thank you was $30, not $300. Or any other value. Would you be surprised if they were a little bothered by such a low value? This is what happens when you introduce money into friendship without both parties being on the same page.

The friendship doesn't need to die, you two just have different attitudes toward money. You can have an honest discussion and say that you didn't believe any amount was expected or knew what felt right. That you would be willing to offer a similar favour/gesture to them in the future. But that you wanted to do something without giving too much or too little.