I think the way to look at what will happen is to consider the incentives on an individual voter. Call him Fred. Like most people Fred votes according to his individual interest. He votes for a party that will give him more money. Many others vote likewise and that party wins. So Fred gets his money, rises above the threshold, and ceases to be a voter. He's definitely gained on the deal; getting richer at the cost of the loss of a mostly symbolic privilege only used occasionally and which made an infinitesimal difference to his circumstances or, indeed, to the result of the election.
Therefore let us assume most people act in the same way as Fred did. Soon there aren't that many people still left in the voting class. Assuming as per the question that the system is administered without significant corruption, they cannot be bribed because any substantial bribe would self-defeatingly lift them out of the category of voters. (A factor that would help to keep the system honest would be that any increase in the wealth of a poor person is difficult to hide. Furthermore there are by now not very many electors left, so they can all be kept under scrutiny.)
So the question becomes, what sort of people voluntarily stay in the voter class? Two groups, those who cannot rise out of it - sad cases unlikely to exercise their right to vote - and those who choose not to. The second group splits into two sub-groups: persons for whom civic duty outweighs the desire for material wealth and persons for whom civic duty outweighs the desire for material wealth and whose politics I don't like, i.e. mad fanatics.
If it persisted, this system might result in some sort of "Rule of the Saints". Three of the many historical examples of rule by an austere and committed minority are (in increasing order of their rule's severity) Edinburgh under the Covenanters, Khomeini's Iran, and the Khmer Rouge. But it tends not to persist.
There is an alternative scenario. Going back to Fred, he votes for the party that promises him more money. As in the earlier version, that party gets in, but this time the party bosses are smarter and Fred is … not so smart. They arrange things so that Fred and people like him always think they are going to soon be given enough money to rise out of the voting class but somehow, for excellent reasons, it never seems to actually happen. Fred and his neighbours get benefits and promises just big enough to keep their hopes up, and to dissuade them, perhaps, from seeking to gain wealth by other than political means. They remain in a state of poverty and dependency comforted by the flattery of politicians when election time comes round. I shall refrain from citing real life examples of this system.