hi there & welcome to the forum.
it could be fungal, does the hair come out easily too with bits of the skin/dandruff attached to it? This is often a sign of fungal.
If guineas are experiencing other issues that are lowering their immune system (which if they have URI's they will be weaker fighting off these) they can be more susceptible to other problems (such as mites, lice, fungal, etc).
what are they bedded on? wood based bedding such as shavings or sawdust can stay damp/moist & fungal issues can be a problem because of this, also the urine reacts with the natural oils in the bedding, which may be a factor in the URI's in the first place.
you may need to reconsider bedding if this is the case it may help-some guineas get reactions to wood based bedding, yet others it makes no difference.
Are they indoor or outdoor piggies & what kind of atmosphere do they live in/whereabouts do you live (country/county) as warmer damp climates usually have more problems with fungal issues-if you could update your profile that would be great.
Have they had any big changes in their routine, been moved, changed, bedding, new addition etc?
are they receiving any medications for their URI's?
@helen105281 is an experienced fungal fighter she uses the Imaverol spray i think & uses it as a dip or on the affected area, but it will take a few treatments over time to work.
is the second picture when its been moisturised as it just looks more wet there.
afraid it may need a vet visit though, because it could be the start of mange, this would need different treatment to fungal. try as Gizzy suggested & see if UK based the pdsa can help out, or if your vet will accept payment in installments seeing as you've had a big bill already.
Oh & if they are new guineas that have been bought recently, then keep your receipts & take them (keep copies) to the store or breeder where you got them & they should pay for the treatments as they sold you unhealthy guineas.
make sure they are still eating well & there are plenty of poops though.
Healing vibes for a speedy recovery. xx