There is no right budget since the competition for keywords differs greatly between markets. The CPC (cost per click) for the keyword “make a website” is much higher in the United States than in Australia. So a $100 a month budget in Mexico can be the equivalent of a $1,000 in the United States, as the average cost per click of the keywords in Mexico is much lower than in the US. Pick a spend level that fits your budget, it just might be “budget-constrained” and not deliver clicks on all the keywords. You can raise the budget later!
Have more questions? Submit a request