I have a noco gb40 which has impressed me with the power it can put out of such a small unit, it will easily start a 3.5 derv engine, even after 6 months on the shelf. These are around 100 usd, but there is the smaller, cheaper gb20 which should meet your budget.

Edit-had it approx 4 years, not exactly a lifetime, put a decent life for a jump starter, and still going strong.