Quote Originally Posted by Боб Уайтман View Post
But if you just hear this expression somewhere in the street, I guess most likely it would sound as "от восемьсот семьдесят одной тысячи рублей" (I underlined the parts which are left undeclined). Although that is considered to be grammatically wrong, a lot of people say that way.
+1

Quote Originally Posted by Боб Уайтман View Post
Maybe you are lucky and you have mostly communicated with well-educated people
I can hardly imagine, let's say, a workshop man, saying "от восьмиста семидясети одной тысячи рублей" with proper declension of all numeric parts.
«Восьмиста» is not the proper form. It’s the form built by analogy with the oblique-case form of 100 «ста». It is also widely used among Russian speakers.

Just in case, I’ve made a tool that declines Russian numerals properly.