You're right on those ads., there are plenty of undeveloped land offerings in El Paso. The important thing here would be to know, what are your needs? If you are looking for a parcel with the intent of future speculation, this are the ones. If you think on building a house on them, not the right ones for you.
El Paso city planning and regulations will not isue a home construction permit on a site that doesn't count with basic services such as water and electricity.
On the other part, we have several parcels for sale in El Paso that have utilities in site and/or, right on the property line.
I hope this would be of any help to you.
I looked at the El Paso MLS and didn't find any 20 acre tracts, but I did find plenty of 5 acre tracts in the $200k's and one that was almost 100 acres! I don't typically see very many El Paso folks here on Trulia. If you would like, I can refer you to someone out there who can better assist you. You may contact me through my profile.