With the TPD requirements, DIY is a bit more difficult than if you were living elsewhere, especially with a target strength of 8mg. The bureaucrats have made it all but impossible to do DIY on the cheap. And those 30ml bottles you're purchasing? TPD rules forbid that.
Max nicotine strength (including base nicotine) is 20mg. Largest bottle you can purchase is supposedly 10ml.
What TPD does (as most if not all regulatory schemes do) is apply across the board, affecting the costs of all alternatives without affecting their rankings. That is, if DIY was the most cost effective plan absent TPD regs, it would still be so in the presence of TPD.
If store-bought liquid costs 13€ and DIY juice costs 4€ pre-TPD, and TPD imposed an additional cost of 10% (financial and economic combined), then store-bought would cost 14.30€ and DIY would cost 4.40€. The only way for the ranking of alternatives to change would be if the TPD scheme hit DIY much harder.