cheapest and the best option, are you using a wall paper stripper by any chance? a steamer?, im in the middle of refitting my kitchen as well and have just scraped 3 walls of old skim off which was painted with oil base eggshell it took me ages reason i done it is because a lot of it wasn't stuck properly, its all about perseverance im sure your old wall paper would succum to a bit of steam from a steamer which you can buy for about £20The cheapest option is to keep scoring the wall paper with a stanly knife and keep sponging it down with warm/hot water, but thats just my opinion.