A比B+形容词+数量补语/程度补语

Think of 比 as a seesaw that already weighs two things against each other. 很 means 'very,' which is a one-sided judgement, and it has no place on that seesaw. When you say 我比他很高, native ears hear something like 'I am more very tall.' Drop 很 and let the adjective do the work: 我比他高. If you want 'a lot,' add 得多 after the adjective instead: 他比我高得多. For the standalone 'very' pattern, see 很 before adjectives.

English puts 'much' in front of the adjective: 'much taller.' Chinese flips it, and the size tag goes at the end. So 'much taller' is 高得多, not 得多高. If you drag 得多 in front of 高, the sentence starts sounding like a question about someone's height. Park the tag last and you're safe every time. Same logic applies to 一点儿: 高一点儿, never 一点儿高.

The chunk after the adjective sets how big the difference is. 一点儿 means the gap is tiny, maybe a fingernail's width. 得多 means the gap is obvious, even dramatic. The grammar is identical, so only the meaning of the gap changes. This matters in real life: telling a coworker 你比我高一点儿 when they tower over you lands as a joke. If you want to soften a comparison without committing to a size, just drop the tag entirely: 他比我高.

Both sentences are correct, but they lean different ways. 便宜得多 sounds standard and slightly written. 便宜很多 is what people actually say while chatting. The one move that wrecks the sentence is stacking them: 便宜得多很多 is like saying 'much much.' Pick one tag and stop. “坐火车比坐飞机便宜得多，去飞机场也要花很多时间，所以，小商觉得坐火车比坐飞机好。” ── 《去上海》