1. She is bargaining with the street vendor over the price.
她与街头小贩讲价钱。
还能这样说：
She is negotiating price with the street vendor.
She is dickering with the street vendor.
应用解析：
big bargain 便宜货;
drive a hard bargain (在某事上)极力讨价还价，坚持苛刻的原则
2. I'd take it if you reduce it to twenty Yuan.
如果20元钱的话我就买。
还能这样说：
I will buy it if it's 20.
I'd take it for twenty Yuan.
应用解析：
take us as you find us 我们就这样，请你将就些吧;
take sb. wrong 误解某人的意思;
take sb. unawares 使某人出其不意，攻其不备;
take out of 取出，除去，要赔偿，报酬
3. How about fifty Yuan?
50元钱怎么样?
还能这样说：
Can I get it for fifty Yuan?
Is it possible for fifty Yuan?
4. I'd buy this if it were cheaper.
再便宜一点我就买。
还能这样说：
I'll take this if you come down a little.
I'd buy this if you reduce the price a little.
应用解析：
a best buy 最合算的买卖;
on the buy 买势很旺;
buy off 收买，贿赂;纳款以免军役;
buy over 收买;行贿;大量收购