With the coinage reform of 348, production at the mint at Cyzicus was cut back so that only 6 officinae were needed. Theta is the number for 9, so there is no chance that this is an uncataloged new officina number. The mangled numeral at the end is not a Theta, but could be a Beta or a Sigma. My money is on "B."
Given the way the inscription spills over the flan, and the type B version of the falling horseman (beardless with hat) I'm guessing the coin diameter is in the range 16-18mm with a weight near 2.45g. Given the reverse inscription break MP-RE and an obverse for Constantius II, I would attribute this as RIC VIII Cyzicus 104 (if pearl diademed) or 105 (if rosette diademed) - pending confirmation of flan size and weight.
RIC VIII Cyzicus 104 officina B:
Constantius II, pearl diademed; falling horseman (type B) for Cyzicus FEL TEMP with the MP-RE break pattern.
Given condition and rarity factor, the value of this example is $5 or less. If cleaned up and shown to be in a better state of preservation it might be worth a bit more, but still has significant problems on both sides.