Over all, chinese millitary are not that under paid at all, comparing to government officers that is. Only goverment officers in costal regions and rich cities are paid much higher than PLA officers, low-mid ranking PLA officers' pay is about the same level as government offcials in relative poorer provinces, but if you consider that the army provide free food, free uniform, free accomodation, free transport, free medicare, free training/education, then its not that bad at all.
More important, most low-mid ranking PLA officers leave the army after a number of years, the army provide them a huge social/business network, solid references, political advantage, new skills etc. An ex army officier has a huge advantage over the long term, he/she will always be considered first for any government related positions and promotions and keep an advantage over someone with equal education/skills but without army experience.
As for the ordinary soldiers, the same advantage applys to them, they are poorly paid, but they are mostly peasants from very poor regions, where they otherwise would be poor anyway, joining the army is for most of them an investement in the future.
High ranking officiers are not poor, they may not wel paid as their western counterparts, but they are paid enough in a country like china.