WASHINGTON — An emerging deal to lower interest rates on student loans hit a major obstacle Thursday after lawmakers were told it carried a $22 billion price tag over the next decade.

The proposal was designed to offer Democrats the promise that interest rates would not reach 10 percent and to give Republicans a link between borrowing terms and the financial markets that they sought. But at that cost, the bipartisan coalition behind it decided to push pause and return to negotiations to bring that cost down.

The estimate was described by a congressional aide involved in the negotiations. The aide was not authorized to discuss the proposal by name and insisted on anonymity because the Congressional Budget Office report had not yet been widely released.

The unexpected cost estimate was unlikely to end talks among lawmakers about how they might reduce rates on subsidized Stafford loans, which doubled to 6.8 percent last week in the wake of congressional inaction.

Efforts to restore those rates to 3.4 percent were abandoned in favor of a new compromise that bears many similarities with a bill that House Republicans have passed, and with President Obama’s budget proposal.

Under the plan lawmakers were considering, interest rates on new loans would be based on the 10-year Treasury note, plus an additional percentage to pay for administrative costs. The proposal includes a limit on how high rates could climb, a provision that Democrats insisted be included in any legislation.

That proved unexpectedly costly and, for the moment, problematic for negotiators.

Undergraduate students would have seen better terms than the current 6.8 percent rate but could have faced higher costs if the economy improves and Treasury notes become more expensive. Rates for students this fall would have been around 4 percent and would have been capped at 8.25 percent in future years.