its nothing to do with your income, thou you/she needs money to finance her visit, you always risk the chance of refusal for a VV for a spouse, because if your married why is your wife only visiting you and not applying for a settlement visa?
the problem is in the past some partners came on visit visa's and tried to stay applying for FLR(m) and then UKBA had to go thru a lengthy process of getting her to leave or appeals.

also if she did stay they would lose their 'entry clearance fee' which is basically the large price difference btw a settlement visa and a VV