In GUNS OF WAR, I define a specific unit type called Mounted Foot, based on Historical Renaissance Dragoons:
Mounted Foot (MF) are multi-role troops that are organized and trained as infantry, but provided with horses for strategic mobility. Their horses enable Mounted Foot to be employed for scouting, screening, foraging, labor, and light skirmishing roles, all at less cost than traditional cavalry. Historical examples of Mounted Foot include early Swedish and English dragoons and later American and British mounted rifles.
Horse Transport: Mounted Foot have the daily movement of light cavalry. One unit of Mounted Foot counts as two units of light cavalry for purposes of reconnaissance rolls (D@W: Campaigns p. XX), supply (D@W: Campaigns, p. XX), and pursuit rolls (D@W: Campaigns p. XX and D@W: Battles p. XX).
Mounted Laborers: Mounted Foot count as skilled laborers for purposes of construction projects (D@W: Campaigns, p. XX).
Variable Deployment: On the battlefield, a unit of Mounted Foot can deploy as either one unit of Drilled Foot or two units of Irregular Mounted. The selection must be made during Deployment and cannot be changed for the duration of the battle. A Mounted Foot unit deployed as two Irregular Mounted units will reform after the battle back into one unit. If either or both IM units took damage, then reduce its total uhp by the percentage of uhp lost combined across the two IM units. Round fractions to the nearest whole number, rounding 0.5 up if odd and down if even.
EXAMPLE: A unit of 120 Mounted Foot with 6 uhp is deployed during a battle as two units of 60 Irregular Mounted, each with 6 uhp. (Remember, half of a typical cavalry unit’s uhp are coming from its mounts). The first unit loses 3 uhp while the second loses 4 uhp. Combined, they have lost a total of 7/12uhp. When the reform as a single unit, the Mounted Foot have lost (7/12 x 6) 3.5 uhp, rounded to 4 uhp. The unit has 2 uhp remaining.
Guns of Wars' dragoons cost 21gp per soldier and require an additional month of training time over normal flintlock riflemen. They have the supply cost of cavalry (4gp/soldier/week). They are mounted on light horses (75gp) and have the same armor and weapons as normal flintlock riflemen plus an extra hand axe (3gp) and saddle, tack, and bags (35gp), for a total increase of 111gp in equipment cost and 9gp in training cost, or 120gp total.
In your case, you are looking at troops with considerably lighter armor (20gp savings) and slower horses (35gp savings), so that the difference in equipment and training cost is 65gp instead of 120gp, e.g. it's about half. Therefore I think you could say that your Anglo-Saxon mounted infantry cost half-way between heavy infantry (12gp) and dragoons (21gp), e.g. 16gp per month.
You'd use the supply cost for cavalry rather than the supply cost for infantry to represent the upkeep on the horses. That's a difference of 0.5gp per soldier for foot per week and 4gp per soldier per week for mounted. Horses are a lot more expensive to maintain in the field. This represents the added cost of a mercenary/soldier with a horse. (And yes that works out very close to 15gp per month you mentioned in your post. The high cost is because horses that are being ridden every day long distances don't have time to graze, and need to be fed expensive nutrient-dense oats, and require frequent swapping, care, etc.)