Citing costs, Georgia and Oklahoma have decided against adopting standardized tests being created by a consortium of states as part of the new Common Core national academic standards.

And politicians in other states — including Indiana and Florida, which has been a leader in the development of the Common Core — are voicing similar concerns, suggesting that more defections could be on the way.

“I’m disappointed to see those states drop off, but I’m not surprised,” said Mitchell Chester, Massachusetts’s education commissioner, who chairs one of the two groups of states that are designing math and reading tests linked to the Common Core standards. Georgia and Oklahoma are members of the 21-state consortium, known as the Partnership for Assessment of Readiness for College and Careers, or PARCC.

“I didn’t expect when we started this journey that all the states that joined at the front end would stick with it,” Chester said.

The Obama administration has invested heavily in the idea of states agreeing to common standards and collaborating on tests. It awarded $330 million to two groups — PARCC and the Smarter Balanced Assessment Consortium — to develop valid, reliable tests that could be administered and compared across state lines.

But on Monday, when PARCC said it would cost states $29.50 a student for both math and reading tests, Georgia had sticker shock. The state, which spends $12 a student for tests in math and reading, said it would instead write its own tests, perhaps joining with other states in a regional effort.

The PARCC tests are more expensive than the multiple-choice “bubble tests” widely used today because they are designed to measure critical thinking skills, requiring students to write analytical essays and demonstrate their understanding of mathematical concepts. The test must be graded by hand and not by computers, adding to the cost, Chester said.

And test questions will be made public after the tests are administered, allowing students to learn from their wrong answers but requiring new questions for the next year.

“We’re designing a very ambitious assessment that sets a high bar, and you can’t do it on the cheap,” Chester said.

Still, for some states, the PARCC tests will be cheaper. Maryland spends $32 a student and the District spends $112 a student, according to a recent study by the Brown Center on Education Policy at the Brookings Institution. Virginia has not adopted the Common Core.

Written by governors and state education officials in both parties and largely funded by the Bill and Melinda Gates Foundation, the Common Core standards are designed to create consistent math and reading standards from kindergarten through 12th grade. Currently, academic standards vary widely among states, and that patchwork nature has been partly blamed for mediocre rankings of U.S. students in international comparisons.

The Common Core standards do not dictate curriculum, allowing states to decide what to teach.

Defections based on cost are the latest headache for the Common Core, which has been fully adopted by 45 states and the District and will be in place by the 2014-15 school year. The standards have been attacked by conservatives and tea party activists, who say they amount to a federal intrusion into local school systems. They are also under fire from some progressives, who don’t like standardized tests and are uncomfortable with the role of the Gates Foundation.

If many more states decide to write their own tests, it can dilute some of the benefits of the Common Core, supporters said.

“We won’t be able to compare their test scores — it’s almost as simple as that,” said Chester E. “Checker” Finn Jr., president of the Fordham Institute, a right-leaning think tank that supports the Common Core. “It’s very difficult, sometimes impossible, to meaningfully compare a score on one test to a score on a different test. You can do some fancy things to get an approximation. But that simple comparison of Springfield, Ohio, with Springfield, Illinois, and Springfield, Mass., would be out of reach.”