The main advantage of a will drawn up by a solicitor is that, in the first instance, you are actually receiving formal legal advice. A testator may believe that their draft will is legally sound. However, a solicitor will advise you on whether or not this is so and may be able to give you additional advice on matters of which you may be unaware.
Secondly, if the solicitor and anyone else in the practice witness the testator's signature that will have credibility in the event that the will is contested. Imagine chasing up Ernie - the fastest milkman in the West - to give evidence that he witnessed the testator's signature one morning on his rounds 25 years ago. No thanks.
Given what is at stake in a will and the utterly horrendous problems which can follow if it is not drawn up validly a fee of €100 is exceptionally good value for advice and drafting.