Constantine I. 306-337 AD. AE 21mm Follis, Rome.

Obv: IMP C CONSTANTINVS PF AVG, laureate, draped, cuirassed bust right.
Rev: MARTI CON-SERVATORI, Mars advancing right, holding
transverse spear and shield. 
Mintmark RS. 

RIC VI Rome 311, rated Rare. 

80 Euros (approx. USD $105.09 GBP £68.18 ) 

Courtesy of tintinna on deamoneta.com, Dec. 2010.