It is not about new cost to replace existing Mig29, but about what to buy once Mig-29 all retire in 10-20 years. Remember, you are up against F-35s of your neighbors and/or potential invaders.
It's a seller's market since there are very few options for affordable 5th gen fighter jets. In fact, FC-31 could be the only one for sale. IMHO, Korea/Japan/Turkey projects all have a very low probability of success. Even if they do somehow manage to build the jet, it could be much inferior to FC-31
The F-35 is indeed very expensive, but JF-31 is not cheap, in fact it is more expensive, why? simply to be cheap and make a profit, needs lots of units sold in a very long period.
Adding two RD-93 means its performance is pretty compromised.
African countries will not buy expensive jets because its highly prohibitive, one aircraft priced at 100 million is not cheap, if you add they need 20 and they need the extra purchases of spare parts, simulators weapons etc etc and flying hours.
Old fighters like the F-15, F-16 or MiG-29 have been produced in the thousands, so the unit price is cheap.
JF-31 can be claimed to be cheap, cheaper than F-35, but it will not make a profit unless it is sold in the hundreds.
I will explain, commercial aircraft, a E-190 is cheaper than a JF-31, in fact you can buy 3 E-195 for a single JF-31 if its price USD $100 million dollars a E-195 cost around USD $35 million dollars.
A commercial jet, every time it flies makes money, military jets do not, So a poor country has more chances of getting S-400s than stealth fighters.
If China does not buy JF-31, the likelihood it will be more successful than other countries is a joke.
For the JF-31 to be successful needs purchases from China like the F-35 has from the USA, compare for example how many F-35 the USA is buying to keep the F-35 cheap for foreign customers.
Japan or South Korea are buying dozens not hundreds, Ethiopia will buy not dozens but 9 -7 aircraft if lucky, poor countries investing in aircraft of 5th generation is almost a dream.
Take for example Argentina or Brazil, two nations of the G-20, how many fighters can afford?
JF-17 has more chances of been successful than JF-31, poor nations will buy old fighters like F-16s or Gripens.
JF-31 will be doomed if they do not get China`s purchases