If anyone is finding Skin & Me or Dermatica expensive, but still wants to use tretinoin, you can buy it from online pharmacies abroad (you just can't get it without a prescription in the UK or Ireland). So in Spain and Greece I believe you can get it over the counter, if you know anyone who lives there and would be kind enough to send it to you - or I buy mine from Farmacia Tristaina, an online pharmacy based in Andorra.
The brand name is Retirides and it comes in 3 strengths - 0.025%, 0,05% and 0.1%. I've been buying from them for a few years and it's been good, it's about €20 per 30g tube and they last me for about 3 months using it 4 or 5 times a week, you only need a pea sized amount.
I would recommend starting with the lower strength and working up slowly. You do have to pay delivery charges of about 20 euro, but overall I would say it works out cheaper than the subscription services. Although you don't get the add ons with other ingredients like niacinamide, hydraquinone etc.
One time when the package didn't get through customs they refunded me the cost of the product once it was returned to them, but I didn't get delivery charges refunded unfortunately. You will get emailed out a tracking link, but it does take a few weeks for delivery. The only thing is, I'm not sure if delivery to the UK would have extra customs charges. I get it delivered to Ireland with no problems, but I usually only buy 3 or 4 tubes at a time to sort of go under the radar.